docs(#3241): unify help generation and lint into vimdoc.sh, generation uses nvim source build to execute

Author: alex-courtis

.github/workflows/ci.yml

@@ -82,6 +82,7 @@ jobs:
           mkdir -p "${DIR_NVIM_SRC}"
           curl -L "https://github.com/neovim/neovim/archive/refs/tags/${{ matrix.nvim_version }}.tar.gz" | tar zx --directory "${DIR_NVIM_SRC}/.."
           cd "${DIR_NVIM_SRC}"
+          make doc
           make lintdoc
 
       - run: make help-check