Add rel links to sitemap and rss feed in blog starter kit

[ttmc]

Changes

This PR fixes #13041

• It edits only one file, BaseHead.astro, in the Astro blog starter kit. It adds three new lines to that file.
• It adds a rel link to the (already-existing) /sitemap-index.xml file created by @astrojs/sitemap
• It adds a rel link to the (already-existing) /rss.xml file created by @astrojs/rss
• The link to the RSS feed file needs the global SITE_TITLE, so I added a line to the frontmatter to import that.

Testing

I created a personal blog using the Astro blog starter kit and noticed that these rel links were missing, so I added them. It worked.

Docs

No additional documentation is required because of this PR. In fact, I got most of the code for this PR from existing Astro documentation:

• the documentation for @astrojs/sitemap
• the documentation for @astrojs/rss

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Latest commit: a20c486

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