Remove all markdown includes

[dyladan]

When looking at generated pull request previews and diffs from respec, they are not working because we use the markdown include (https://respec.org/docs/#external-includes) which fails because the browser cannot find the markdown file to include when previewing a PR. Fixing this will require pulling the full specification into a single index.html file.

This issue is to track the overall work, but I propose we do this section-by-section to ensure nothing is accidentally changed while making the transition. Doing a single section at a time will also prevent PRs from having to do large conflict resolutions. It will also be a good time to review respec best practices.

Before:

After: