feat: adds support for specifying a blog post's updated date

[julien-deramond]

Describe the pull request

This PR adds an optional new updateDate field for blog posts to define the last update that happened.

Why

This information can be interesting for readers when blog posts have been updated after feedback from the readers, typos, changes of URL, or adaptation for whatever reason.

How

• The schema at packages/starlight-blog/schema.ts has been updated to add a new updateDate field which is an optional date
• It is considered as a metadata information so packages/starlight-blog/libs/content.ts has been modified to include this new field
• In terms of rendering, this updateDate value has been treated like the date value and is handled in packages/starlight-blog/components/Metadata.astro:
  • Being secondary information, I chose arbitrarily to display it with a small size via var(--sl-text-xs);
  • Initially, I tried to display it under the publication date, but the rendering was weird. Putting it after the authors seem more elegant, even if maybe we lose the logical link between the 2 dates
  • Please note that this updateDate is not rendered if it's not defined, or that its content has the same value as the publication date
• Testing:
  • "visual tests" has been added in docs/src/content/docs/blog/vario-nunc-polo.md and docs/src/content/docs/blog/sequantur-quaeritis-tandem.md
  • I wasn't sure if it was necessary to add e2e tests that didn't seem appropriate for that. Please tell me if some must be added
• RSS: IMHO there's no impact on the RSS part since there's no update date defined in https://www.w3schools.com/xml/xml_rss.asp#rssref. It only exists as lastBuildDate for the channels. Plus, I don't think we need to modify the pubDate to use the updateDate as it would modify the order of appearance of the blog posts, even if it is mentioned that pubDate is supposed to be the "last-publication" date for the item. I'm not 100% sure about it, but I would handle it like this.

Some links

• Updated documentation: https://starlight-blog-docs-git-fork-julien-der-df4974-hideoos-projects.vercel.app/guides/frontmatter/
• Demo blog: https://starlight-blog-docs-git-fork-julien-der-df4974-hideoos-projects.vercel.app/blog
  • "Sequantur quaeritis tandem" has the same updateDate as the date, so the updateDate is not rendered
  • "Vario nunc polo" has a different updateDate, so it is rendered
  • The other blog posts don't have any updateDate, so it's not rendered

Screenshots

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Name	Status	Preview	Comments	Updated (UTC)
starlight-blog-docs	✅ Ready (Inspect)	Visit Preview	💬 Add feedback	Jul 2, 2024 4:20pm