Support component customisation (#709)

Co-authored-by: Sarah Rainsberger <sarah@rainsberger.ca>
Co-authored-by: HiDeoo <494699+HiDeoo@users.noreply.github.com>

.changeset/plenty-donkeys-lay.md

@@ -0,0 +1,21 @@
+---
+'@astrojs/starlight': minor
+---
+
+Add support for overriding Starlight’s built-in components
+
+⚠️ **BREAKING CHANGE** — The page footer is now included on pages with `template: splash` in their frontmatter. Previously, this was not the case. If you are using `template: splash` and want to continue to hide footer elements, disable them in your frontmatter:
+
+```md
+---
+title: Landing page
+template: splash
+# Disable unwanted footer elements as needed
+editUrl: false
+lastUpdated: false
+prev: false
+next: false
+---
+```
+
+⚠️ **BREAKING CHANGE** — This change involved refactoring the structure of some of Starlight’s built-in components slightly. If you were previously overriding these using other techniques, you may need to adjust your code.

docs/src/content/docs/guides/overriding-components.md

@@ -0,0 +1,133 @@
+---
+title: Overriding Components
+description: Learn how to override Starlight’s built-in components to add custom elements to your documentation site’s UI.
+sidebar:
+  badge: New
+---
+
+Starlight’s default UI and configuration options are designed to be flexible and work for a range of content. Much of Starlight's default appearance can be customized with [CSS](/guides/css-and-tailwind/) and [configuration options](/guides/customization/).
+
+When you need more than what’s possible out of the box, Starlight supports building your own custom components to extend or override (completely replace) its default components.
+
+## When to override
+
+Overriding Starlight’s default components can be useful when:
+
+- You want to change how a part of Starlight’s UI looks in a way not possible with [custom CSS](/css-and-tailwind/).
+- You want to change how a part of Starlight’s UI behaves.
+- You want to add some additional UI alongside Starlight’s existing UI.
+
+## How to override
+
+1. Choose the Starlight component you want to override.
+   You can find a full list of components in the [Overrides Reference](/reference/overrides/).
+
+   This example will override Starlight’s [`SocialIcons`](/reference/overrides/#socialicons) component in the page nav bar.
+
+2. Create an Astro component to replace the Starlight component with.
+   This example renders a contact link.
+
+   ```astro
+   ---
+   // src/components/EmailLink.astro
+   import type { Props } from '@astrojs/starlight/props';
+   ---
+
+   <a href="mailto:houston@example.com">E-mail Me</a>
+   ```
+
+3. Tell Starlight to use your custom component in the [`components`](/reference/configuration/#components) configuration option in `astro.config.mjs`:
+
+   ```js {9-12}
+   // astro.config.mjs
+   import { defineConfig } from 'astro/config';
+   import starlight from '@astrojs/starlight';
+
+   export default defineConfig({
+     integrations: [
+       starlight({
+         title: 'My Docs with Overrides',
+         components: {
+           // Override the default `SocialLinks` component.
+           SocialIcons: './src/components/EmailLink.astro',
+         },
+       }),
+     ],
+   });
+   ```
+
+## Reuse a built-in component
+
+You can build with Starlight’s default UI components just as you would with your own: importing and rendering them in your own custom components. This allows you to keep all of Starlight's basic UI within your design, while adding extra UI alongside them.
+
+The example below shows a custom component that renders an e-mail link along with the default `SocialLinks` component:
+
+```astro {4,8}
+---
+// src/components/EmailLink.astro
+import type { Props } from '@astrojs/starlight/props';
+import Default from '@astrojs/starlight/SocialIcons.astro';
+---
+
+<a href="mailto:houston@example.com">E-mail Me</a>
+<Default {...Astro.props}><slot /></Default>
+```
+
+When rendering a built-in component inside a custom component:
+
+- Spread `Astro.props` into it. This makes sure that it receives all the data it needs to render.
+- Add a [`<slot />`](https://docs.astro.build/en/core-concepts/astro-components/#slots) inside the default component. This makes sure that if the component is passed any child elements, Astro knows where to render them.
+
+## Use page data
+
+When overriding a Starlight component, your custom implementation receives a standard `Astro.props` object containing all the data for the current page.
+This allows you to use these values to control how your component template renders.
+
+For example, you can read the page’s frontmatter values as `Astro.props.entry.data`. In the following example, a replacement [`PageTitle`](/reference/overrides/#pagetitle) component uses this to display the current page’s title:
+
+```astro {5} "{title}"
+---
+// src/components/Title.astro
+import type { Props } from '@astrojs/starlight/props';
+
+const { title } = Astro.props.entry.data;
+---
+
+<h1 id="_top">{title}</h1>
+
+<style>
+  h1 {
+    font-family: 'Comic Sans';
+  }
+</style>
+```
+
+Learn more about all the available props in the [Overrides Reference](/reference/overrides/#prop-types).
+
+### Only override on specific pages
+
+Component overrides apply to all pages. However, you can conditionally render using values from `Astro.props` to determine when to show your custom UI,  when to show Starlight’s default UI, or even when to show something entirely different.
+
+In the following example, a component overriding Starlight's [`Footer`](/reference/overrides/#footer-1) displays "Built with Starlight 🌟" on the homepage only, and otherwise shows the default footer on all other pages:
+
+```astro
+---
+// src/components/ConditionalFooter.astro
+import type { Props } from '@astrojs/starlight/props';
+import Default from '@astrojs/starlight/Footer.astro';
+
+const isHomepage = Astro.props.slug === '';
+---
+
+{
+  isHomepage ? (
+    <footer>Built with Starlight 🌟</footer>
+  ) : (
+    <Default {...Astro.props}>
+      <slot />
+    </Default>
+  )
+}
+```
+
+Learn more about conditional rendering in [Astro’s Template Syntax guide](https://docs.astro.build/en/core-concepts/astro-syntax/#dynamic-html).

docs/src/content/docs/guides/sidebar.mdx

@@ -1,8 +1,6 @@
 ---
 title: Sidebar Navigation
 description: Learn how to set up and customize your Starlight site’s sidebar navigation links.
-sidebar:
-  badge: New
 ---
 
 import FileTree from '../../../components/file-tree.astro';

docs/src/content/docs/index.mdx

@@ -5,6 +5,8 @@ head:
     content: Starlight 🌟 Build documentation sites with Astro
 description: Starlight helps you build beautiful, high-performance documentation websites with Astro.
 template: splash
+editUrl: false
+lastUpdated: false
 banner:
   content: |
     <span class="ph-banner">

docs/src/content/docs/reference/configuration.md

@@ -436,3 +436,19 @@ Sets the delimiter between page title and site title in the page’s `<title>` t
 
 By default, every page has a `<title>` of `Page Title | Site Title`.
 For example, this page is titled “Configuration Reference” and this site is titled “Starlight”, so the `<title>` for this page is “Configuration Reference | Starlight”.
+
+### `components`
+
+**type:** `Record<string, string>`
+
+Provide the paths to components to override Starlight’s default implementations.
+
+```js
+starlight({
+  components: {
+    SocialLinks: './src/components/MySocialLinks.astro',
+  },
+});
+```
+
+See the [Overrides Reference](/reference/overrides/) for details of all the components that you can override.