New content (& content collections) section for 5.0

This PR:

• updates the content collections guide, splitting it into a main landing page and two smaller guides
• updates Markdown guide for freshness, streamlined-ness, and simplifies by removing MDX content (which has, already in live published docs, been moved to the MDX page itself)
• updates a bunch of links that have shifted during the flight and other minor tweaks to give an overall "Content" refresh

Re: Content Collections (now, powered by a layer!) for @ascorbic

1. Please review the three new content collections guide pages preview here:
   content-collections.mdx
   defining-collections.mdx
   working-with-collections.mdx

You can ignore everything else! It's just links etc. around the rest of the site because the original page (and a couple of other related content / markdown pages) have changed. There's also some future proofing for when collections don't have to be in src/content/

By "review" I mean:

• correct any inaccuracies!
• point out any wrong nuances, takes, or nudges that don't feel right to you
• is anything key to using collections missing? We can always add more over time, but is there anything that's absolutely needed here or people are just not going to be successful?
• if you happen to notice language stuff, then all proofreading, grammar, formatting etc. are fine here, but it's not required to take a specific pass or pay super close attention to this since there will still be lots of available eyes to catch that stuff.

My main goal is to make sure that this guide faithfully and accurately represents both how to get set up and use collections, and the "vision" for collections.

After, outside of this PR:

2. I have added in this PR a shell for a new Loader API page. It seems like this should be a good place to document the Loader API in technical detail, and describe how to build one. You don't need to contribute to this branch. Once the guide content is fine, we can merge this branch and that shell page will exist. You can separately make a PR that will include:

• content for the Loader API page
• any updates to the API reference page, content collections section

Those two can be together in the same PR if you want, to get all the API content squared away.

My first priority is to get this PR merged into the beta branch so I can start fixing any conflicts because there were a lot of moving pieces in breaking up the content collections page! The API material can be added later!