Enable specifying a Markdown description of a category

[parlough]

Then we can add descriptions on the API docs overview/index page: https://api.dart.dev/main

This would allow us to explain what the categories actually mean and why they exist. For example:

• That "Core" libraries should work across all platforms
• That "VM" libraries only work when ran or compiled for native platforms
• That usage of "WEB (LEGACY)" libraries should be migrated (and what to)
  • This is the primary use case envisioned here, similar to add new items to categories #3617
• Perhaps more :)

While a normal string would be sufficient, supporting Markdown here would be great as it would enable easy links.

[parlough]

\cc @devoncarew Since you're modifying the categories now, what do you think?

This could show up below the relevant headers on the index page:

[devoncarew]

Yeah, I think this would be great - it would give more context to the categories.

We can manually describe things in the markdown above the generated list, but messaging things more than once can be useful.

[srawlins]

I think I got mixed up, but this is still not implemented, right? I think @devoncarew landed something separate, which enabled package:web to be listed in the Web category of the Dart SDK, right?