Problem:
- The layout of the doxygen HTML "dev docs" has been broken for 1+ years.
- There is no evidence that anyone uses the doxygen HTML.
- In the meantime since we introduced the doxygen HTML pages, the Nvim
  C docstrings have been surfaced in other ways:
  - Nvim LSP client
  - Generated :help docs like ":help api" and ":help ui".

Solution:
- Remove `contrib/doxygen/` from neovim repo.
  - Note: keep `src/Doxyfile`, it is used by `scripts/gen_vimdoc.py`. ☠️
- Remove `doc/dev/` from website. https://github.com/neovim/neovim.github.io/pull/291
- Remove doxygen job from the neovim/doc repo. https://github.com/neovim/doc/pull/28
- Future/ongoing: ":help dev" and other :help docs will continue to be
  improved as the "source of truth" for developer-related docs.

ref https://github.com/neovim/neovim/pull/824