Create features.md

[bamurtaugh]

Add the content from https://github.com/devcontainers/spec/blob/main/proposals/devcontainer-features.md.

One to-do is to make the headers into links (i.e. ## <a href="#contribution-approaches" name="contribution-approaches" class="anchor"> Contribution approaches </a>), but that could also be done in a subsequent PR.

[Chuxel]

Yeah I think this would be reasonable to add at this point along with a nav! We can then link to it from the devcontainer.json reference and the main spec. A few thoughts:

1. Maybe we should add [Proposal] to the heading with a brief statement saying something like "This section provides information on a currently active proposal. See Dev Container Features spec#61 for input and links to other proposed improvements. "?
2. It looks like this references the distribution proposal directly. We may want to pull that file in at the same time or adjust the link to point to the markdown.
3. We could reference the features section from the main spec I think just by adding a heading under "Other options". E.g., "Dev container 'features' are self-contained, shareable units of installation code and development container configuration. They are applied to container images as a secondary build step and can affect a number of dev container configuration settings. See for more details."
4. In the "features" property in the devcontainer.json reference, we can reference the "devcontainer.json properties" section of this doc for more info at this point.

[bamurtaugh]

Thanks for the great feedback, @Chuxel!

I believe I addressed the 4 points above:

1. Added the word 'Proposal' and a sentence of context to the features doc
2. Pulled in the features distribution doc directly, also using 'Proposal' and the context sentence above
3. Added a features section under "Other Options" in the main spec, linking to the new features.md
4. Added a link to features.md to the features section of the devcontainer.json reference

I also made all the headers in the features and features distribution docs into links for easier sharing and navigation, consistent with the other pages of the site.

Please let me know if the changes are in-line with what you were thinking.

[Chuxel]

A few suggested tweaks.

[bamurtaugh]

Thanks for the suggestions! Opened devcontainers/spec#69 with the Features wording suggestion you mentioned.

I'll merge these changes in as it seems this is a good baseline, and we can continue with other changes folks might have in mind in follow-up PRs.