Document experimental JS API (#4234)

Co-authored-by: Sarah Rainsberger <sarah@rainsberger.ca>

src/content/docs/en/reference/cli-reference.mdx

@@ -373,3 +373,129 @@ Automatically opens the app in the browser on server start.
 ### `--help`
 
 Prints the help message and exits.
+
+## Advanced APIs (Experimental)
+
+If you need more control when running Astro, the `"astro"` package also exports APIs to programmatically run the CLI commands. 
+
+These APIs are experimental and their API signature may change. Any updates will be mentioned in the [Astro changelog](https://github.com/withastro/astro/blob/main/packages/astro/CHANGELOG.md) and the information below will always show the current, up-to-date information.
+
+### `AstroInlineConfig`
+
+The `AstroInlineConfig` type is used by all of the command APIs below. It extends from the user [Astro config](/en/reference/configuration-reference/) type:
+
+```ts
+interface AstroInlineConfig extends AstroUserConfig {
+	configFile?: string | false;
+	mode?: "development" | "production";
+	logLevel?: "debug" | "info" | "warn" | "error" | "silent";
+}
+```
+
+#### `configFile`
+
+<p>
+
+**Type:** `string | false`<br />
+**Default:** `undefined`
+</p>
+
+A custom path to the Astro config file. 
+
+If this value is undefined (default) or unset, Astro will search for an `astro.config.(js,mjs,ts)` file relative to the `root` and load the config file if found.
+
+If a relative path is set, it will resolve based on the current working directory.
+
+Set to `false` to disable loading any config files.
+
+The inline config passed in this object will take highest priority when merging with the loaded user config.
+
+#### `mode`
+
+<p>
+
+**Type:** `"development" | "production"`<br />
+**Default:** `"development"` when running `astro dev`, `"production"` when running `astro build`
+</p>
+
+The mode used when building your site to generate either "development" or "production" code.
+
+#### `logLevel`
+
+<p>
+
+**Type:** `"debug" | "info" | "warn" | "error" | "silent"`<br />
+**Default:** `"info"`
+</p>
+
+The logging level to filter messages logged by Astro.
+
+- `"debug"`: Log everything, including noisy debugging diagnostics.
+- `"info"`: Log informational messages, warnings, and errors.
+- `"warn"`: Log warnings and errors.
+- `"error"`: Log errors only.
+- `"silent"`: No logging.
+
+### `dev()`
+
+**Type:** `(inlineConfig: AstroInlineConfig) => AstroDevServer`
+
+Similar to [`astro dev`](#astro-dev), it runs Astro's development server.
+
+```js
+import { dev } from "astro";
+
+const devServer = await dev({
+  root: "./my-project",
+});
+
+// Stop the server if needed
+await devServer.stop();
+```
+
+### `build()`
+
+**Type:** `(inlineConfig: AstroInlineConfig) => void`
+
+Similar to [`astro build`](#astro-build), it builds your site for deployment.
+
+```js
+import { build } from "astro";
+
+await build({
+  root: "./my-project",
+});
+```
+
+### `preview()`
+
+**Type:** `(inlineConfig: AstroInlineConfig) => AstroPreviewServer`
+
+Similar to [`astro preview`](#astro-preview), it starts a local server to serve your static `dist/` directory.
+
+```js
+import { preview } from "astro";
+
+const previewServer = await preview({
+  root: "./my-project",
+});
+
+// Stop the server if needed
+await previewServer.stop();
+```
+
+### `sync()`
+
+**Type:** `(inlineConfig: AstroInlineConfig) => number`
+
+Similar to [`astro sync`](#astro-sync), it generates TypeScript types for all Astro modules
+
+```js
+import { sync } from "astro";
+
+const exitCode = await sync({
+  root: "./my-project",
+});
+
+process.exit(exitCode)
+```