Restructure for API only

[JoelPasvolsky]

This draft PR is just for feedback on the proposed structure, it won't be merged. The final PR will also need to update xrefs.
It enables local builds for testing, using the same theme as the SDK.

It (and most repos) would look like this in the SDK docs:

For simpler repos (dwave-inspector here), we can always have at least one page:

Or (penaltymodel here) we can either directly go to the API:

I like always having at least one underlying page for consistency and aesthetics but some people prefer one fewer click.
I can make draft PRs for the penaltymodel branch and dwave-inspector branch if that is helpful.
For now I'm taking advantage of GitHub's perfectly decent rendition of release notes but later if we want to spend the time working around reno's duplication of anchors between repos to prevent dozens of warnings, we can generate HTML for those again.