update developer docs build config

[adam-enko]

Update MkDocs tasks to improve compatibility with Configuration Cache.

• use com.pswidersk.python-plugin (although it's less capable than ru.vyarus.mkdocs and requires more manual setup, ru.vyarus.mkdocs is not Configuration Cache compatible)
• Specify requirements.txt (helps with reproducible builds, Dependabot)
• Specify the output directory site_dir: build/mkdocs explicitly. Move config and site files so they're not in a subdirectory, and so the generated site_dir doesn't have to use confusing relative paths)
• Added dark/light mode toggle (an unrelated change, but one that helped my eyes during development)
• Define tasks for installing & building mkdocs.
• Update README with the new tasks.
• If possible, log link to site using IntelliJ built-in server (Because the 'serve' command doesn't work with com.pswidersk.python-plugin due to a Gradle issue that doesn't kill spawned processes when the Gradle build ends.)

The output of the build is still in the same location, so the only modification to the GitHub Actions is to the task names.

TODO

• [x] Re-implement multiversion support & redirecting index.html