Update links and headers

[bamurtaugh]

• Make the titles in spec.md and reference.md into links, for easier sharing and consistency (other pages on the site use this format)
• Update broken links in https://containers.dev/implementors/spec
• Add @edgonmsft's content from https://github.com/devcontainers/spec/pull/39/files
• Add info on contributing schema hints/IntelliSense
  • Once we're happy with this wording, I can open a PR to update https://github.com/devcontainers/spec/blob/main/contributing.md

cc @Chuxel

[Chuxel]

Made some additional comments on devcontainers/spec#39

For the schema - do we want people doing a PR to VS Code or the spec repo once we do devcontainers/spec#14 ? (and should we do that at this point is another related question)

//cc @chrmarti on the schema

[bamurtaugh]

Thanks @Chuxel! I'll keep an eye on that PR and add further updates here.

For the schema - do we want people doing a PR to VS Code or the spec repo once we do devcontainers/spec#14 ? (and should we do that at this point is another related question)

Ah good point. I think a PR to the spec repo would be best as it's tool agnostic. In which case, moving the schema now makes sense to me, if @chrmarti agrees. This could at least account for part of devcontainers/spec#14, if we needed to wait or discuss a bit more the schema store part.

Do we need to advise users to update anything in VS Code to get sufficient IntelliSense? Based on @chrmarti's comment: One disadvantage might be that other editors will not offer IntelliSense for the VS Code and Codespaces parts, but that might be fine, it sounds like this would be something for other editors to account for, but not something we'd need to provide updates for in VS Code.

[bamurtaugh]

Added all the latest from @edgonmsft's PR devcontainers/spec#39.

Regarding the contributions.md update to describe updating the schema: I propose we remove that info until we move the schema to the spec repo (part of devcontainers/spec#14), otherwise it feels less VS Code agnostic to direct people to the VS Code repo. I've removed that content now, and I'll proceed with merging this PR.