Revamped RSS instructions for content collections

[bholmesdev]

What kind of changes does this PR include?

• New or updated content

Description

• Updates for [RSS] Get ready for content collections astro#5851
• Change import.meta.env.SITE to new context.site
• Update leading example to use content collections
• Add new "using content collections" section to "Generating items"
• Update glob import instructions to recommend the pagesGlobToRssItems() function
• Update "including full post content" with content collections instructions. The markdown-it recommendation may change once we support an html output for content collections!

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[sarah11918]

Yay! Better RSS docs! 🥳

Thanks for making these so easy to read and follow, Ben! I have some suggestions for tightening up and orienting these to be a bit more usage-based for you to consider, but these should be quick to wrap up!

[sarah11918]

🥳 !

You'll need to re-we in the README itself, too, but they're both good to go!