[RFC] Move developer docs to main docs (or at least to an .rst tree)

## description

At the moment there are a lot of useful dev docs/info spread through files in the repo.

I believe there are several advantages to centralizing the developer docs and bringing them into the main documentation

- we have various ci/checks etc that are only applied to the doc/rst tree
- centralizing docs would decrease the learning time/curve for new developers to find relevant documentation/adopt related standards etc
- one set of documents to maintain (altho i think there is an argument for maintaining the dev docs in a separate tree - but then 2)

we could still leave stubs/links in README files or similar

perhaps this pertains differently to code documentation in the actual source etc than to eg ci/tooling/sandbox docs - but even then there maybe a similar argument to collate the developer docs.