Remove `nvim-linux64.tar.gz` and `nvim.appimage` as maintaining
these is too much work.

Also fix directory names to be consistent.

Problem: No releases for ARM Linux.

Solution: Provide appimages and tarballs for `linux-arm64`. Rename
x86 releases to `linux-x86_64` for consistency.

Also use git tag archive over commit sha if possible.

Simplify usage and remove redundant flags and code.

Problem:

Whether an option is allowed to be empty isn't well defined and
isn't properly checked.

Solution:

- For non-list string options, explicitly check the option value
  if it is empty.
- Annotate non-list string options that can accept an empty value.
  - Adjust command completion to ignore the empty value.
- Render values in Lua meta files

* docs: drop "lua-" prefix from most treesitter tags
* docs: move mouse section from tui.txt to gui.txt
* docs: misc