Problem:
Nvim has Lua but the "nvim" CLI can't easily be used to execute Lua
scripts, especially scripts that take arguments or produce output.

Solution:
- support "nvim -l [args...]" for running scripts. closes #15749
- exit without +q
- remove lua2dox_filter
- remove Doxyfile. This wasn't used anyway, because the doxygen config
  is inlined in gen_vimdoc.py (`Doxyfile` variable).
- use "nvim -l" in docs-gen CI job

Examples:

    $ nvim -l scripts/lua2dox.lua --help
    Lua2DoX (0.2 20130128)
    ...

    $ echo "print(vim.inspect(_G.arg))" | nvim -l - --arg1 --arg2
    $ echo 'print(vim.inspect(vim.api.nvim_buf_get_text(1,0,0,-1,-1,{})))' | nvim +"put ='text'" -l -

TODO?
  -e executes Lua code
  -l loads a module
  -i enters REPL _after running the other arguments_.

Use `white-space: pre-wrap` to preserve white space as per `pre`, but to
allow line wrapping if the display runs out of horizontal space.

This prevents lines overflowing their box, and causing horizontal
scrolling across the entire page on small screens.

This `pre-wrap` technique is used by GitHub to format code for mobile.

See https://developer.mozilla.org/en-US/docs/Web/CSS/white-space#pre-wrap

Co-authored-by: Justin M. Keyes <justinkz@gmail.com>

- https://github.com/neovim/tree-sitter-vimdoc v1.2.4 eliminates most
  errors in pi_netrw.txt, so we can remove that workaround from
  ignore_parse_error().
- improved codeblock