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.

doc/nuwiki.txt

@@ -50,13 +50,11 @@ Highlights:
 ==============================================================================
 2. REQUIREMENTS                                         *nuwiki-requirements*
 
-  * Neovim 0.11+                       (preferred path)
-  * Vim 9.1+ with one of:
-      - vim-lsp     (prabirshrestha/vim-lsp)
-      - coc.nvim    (neoclide/coc.nvim)
-  * `curl` and `tar` (only for the binary download path; not needed if
-    you build from source)
-  * `cargo` and a Rust toolchain (only if you opt into source builds)
+   * Neovim 0.11+                       (preferred path)
+   * Vim 9.1+ with one of:
+       - vim-lsp     (prabirshrestha/vim-lsp)
+       - coc.nvim    (neoclide/coc.nvim)
+   * `curl` and `tar` (for the binary download path)
 
 ==============================================================================
 3. INSTALLATION                                         *nuwiki-installation*
@@ -83,16 +81,8 @@ lazy.nvim: >
     \ 'build': 'vim -e -s -c "source scripts/download_bin.vim" -c "q"',
     \ })
 
-<To force a build from source (skip the download path), set: >
-
-  let g:nuwiki_build_from_source = 1
-
-<in Vim before re-running the build hook, or in Neovim: >
-
-  vim.g.nuwiki_build_from_source = 1
-  require('nuwiki').install()
-
-<==============================================================================
+<Note: the LSP binary is downloaded automatically. No cargo or Rust
+toolchain is required.==============================================================================
 4. CONFIGURATION                                              *nuwiki-config*
 
 Neovim users call `setup()` (omit the block entirely to take every
@@ -380,12 +370,10 @@ Vim-specific globals ~
     middle-click `:badd`s the link target, etc.). Mirrors
     `mappings.mouse = true` in Neovim.
 
-                                                  *g:nuwiki_build_from_source*
-`g:nuwiki_build_from_source`   0
-    When 1, the install step always builds the server with `cargo` and
-    skips the release-asset download.
+*g:nuwiki_no_calendar*
+`g:nuwiki_no_calendar`         0
 
-                                                         *g:nuwiki_binary_path*
+                                                          *g:nuwiki_binary_path*
 `g:nuwiki_binary_path`
     Override the auto-discovered server binary path. If set and the
     file is readable, it overrides the `{plugin}/bin/nuwiki-ls` default.