Add extend options to docs and i18n schemas

[delucis]

Description

• Adds an extend option to both the docs and i18n content collection schemas, which allows users to add additional fields to the default schemas.
• Adds docs for for the docs schema to the frontmatter reference. Added docs:
  • New frontmatter section in Authoring Markdown content guide
  • New extending docs schema reference
  • New extending i18n schema docs (annoyingly Vercel seems not to be rebuilding? So no preview for this one just yet)

[changeset-bot]

🦋 Changeset detected

Latest commit: 79400ff

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

This PR includes changesets to release 1 package

Name	Type
@astrojs/starlight	Minor

Not sure what this means? Click here to learn what changesets are.

Click here if you're a maintainer who wants to add another changeset to this PR

[vercel]

The latest updates on your projects. Learn more about Vercel for Git ↗︎

Name	Status	Preview	Updated (UTC)
starlight	✅ Ready (Inspect)	Visit Preview	Nov 29, 2023 7:35pm

[astrobot-houston]

size-limit report 📦

Path	Size
/index.html	5.12 KB (0%)
/_astro/*.js	20.11 KB (0%)
/_astro/*.css	13 KB (0%)

[bholmesdev]

We schemust merge this right away!

[HiDeoo]

Tested it with my blog integration which ships a custom schema, works as I expected.

Will happily make the following changes to my docs ^^

  import { docsSchema, i18nSchema } from '@astrojs/starlight/schema'
  import { defineCollection } from 'astro:content'
- import { docsAndBlogSchema } from 'starlight-blog/schema'
+ import { blogSchema } from 'starlight-blog/schema'

  export const collections = {
-   docs: defineCollection({ schema: docsAndBlogSchema }),
+   docs: defineCollection({ schema: docsSchema({ extend: blogSchema }) }),
    i18n: defineCollection({ type: 'data', schema: i18nSchema() }),
  }

I would have approved but small question: why are the changes of the i18n guide not visible in the preview? I see the changes in the diff but not in the preview. 🤔

Edit: nvm, they display fine locally, I guess it's a preview issue, weird

[delucis]

why are the changes of the i18n guide not visible in the preview?

Yes, noted this in the PR comment — for some reason Vercel didn’t pick newer commits so the preview is out of date. Don’t really know why!