Move proposals and add documentation

[bamurtaugh]

Fixes https://github.com/devcontainers/spec/issues/338

As @samruddhikhandale proposed, we should move implemented proposals out of the "proposals" folder. I've made several changes in this PR:

• Move implemented proposals into the "specs" folder
• Update each implemented proposal to include a link to its initial discussion issue and where it was merged into the CLI or schema
• Add README content to the "specs" and "proposals" folder with more info on this process
  • In the "specs" README, included a list of the main top-level specs for easier navigation (i.e. overall dev container, Features, and Templates specs)
• Misc polish (i.e. caps, formatting)

I considered moving all of the proposals out of their individual files and instead into the existing spec files, but this felt less ideal:

• It'd make the existing spec files much longer
  • Keeping a set of smaller individual files may better help with quick navigation
• Each file can serve as a point in time reference to that proposal, including the discussion issue and implementation PR links

But if folks would prefer this ^, I'm happy to discuss further.

[samruddhikhandale]

I considered moving all of the proposals out of their individual files and instead into the existing spec files, but this felt less ideal:

Makes sense to me! 👍

[bamurtaugh]

Thank you for the quick review @samruddhikhandale! And for the great idea that spurred this PR 😄. I'll merge now and happy to iterate in future PRs if other folks have additional thoughts.

[113ll3]

bamurtaugh/move-props

[113ll3]

.patch