PageHeader: Add docs to primer.style and features to Storybook

[broccolinisoup]

Closes #1570

This PR adds mdx documentation for PageHeader and export the component in the draft bundle

Merge checklist

• Added/updated tests
• Added/updated documentation
• Tested in Chrome

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[changeset-bot]

🦋 Changeset detected

Latest commit: 55cfa00

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 1 package

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@primer/react	Minor

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[github-actions]

size-limit report 📦

Path	Size
dist/browser.esm.js	87.71 KB (+3.27% 🔺)
dist/browser.umd.js	88.37 KB (+3.3% 🔺)

[mperrotti]

I'd restructure the sections to follow what we have for the other components (for example, ActionList):

## Examples

### With a title

### With a title and actions

<!-- all other examples -->

## Props

### PageHeader

Optional description of the component

<PropsTable>
<!-- props table content -->
</PropsTable>

### PageHeader.ContextArea

Optional description of the component

<PropsTable>
<!-- props table content -->
</PropsTable>

<!-- all other child elements -->

[mperrotti]

Instead of trying to explain the usage details of PageHeader components in the API docs, this should be covered in PageHeader interface guidelines in https://primer.style/design/. Sadly, we don't have these written yet.

When we provide a thorough set of examples, engineers can see how the parts of a PageHeader are meant to be used in live demos.

[mperrotti]

Here's what I'd recommend as a thorough set of examples:

## Examples

### Just has a title

### Title variant="large"

### Title with leading and trailing visuals

### With actions

### With description slot

### With navigation slot

### With a ContextArea with all possible children (only visible on mobile)

### With a ContextArea with all possible children (always visible)

[broccolinisoup]

Thank you so much @mperrotti! This is great. You are right, the initial structure I was thinking is more of an interface guideline content not so much API docs. I updated the docs accordingly and added storybook features as well. Looking forward to hearing your thoughts 🙌🏼