Remove Rust code, delegate server to nuwiki-rs repo

Delete all Rust crates (crates/) and Cargo files. The nuwiki-ls binary
is now built in the separate nuwiki-rs repository and downloaded at
install time from the Gitea release assets, with a cargo build fallback
that clones nuwiki-rs.

Update all documentation to reflect the split repo layout.

autoload/nuwiki/commands.vim

@@ -1175,7 +1175,7 @@ endfunction
 " Mirrors `lua/nuwiki/commands.lua`'s `align_table_at` so plain Vim
 " buffers get the same "tighten columns as cells grow" behaviour the
 " Neovim path enjoys, without needing a vim-lsp roundtrip. Algorithm
-" matches `crates/nuwiki-lsp/src/commands.rs::ops::render_aligned_table`.
+" matches `nuwiki-lsp/src/commands.rs::ops::render_aligned_table` (nuwiki-rs repo).
 
 function! s:is_table_row(line) abort
   return a:line =~# '^\s*|.*|\s*$'