docs: New documentation using mdbook (#471)

* docs: mdbook init

* Separate sub-options into their section

* docs: enable fold

* docs: merge core options into a single section

* doc generation: fix submodules index pages

* docs: add contributing section

* docs: rename 'core' group to 'Neovim Options'

docs: removed the index pages of empty sections

docs: remove obsolete 'mergeFunctionResults' function

* docs: use nix syntax highlighting

* docs: point to the new repo url

* docs: use recursive generation
docs: split submodules into subsections

* docs: fix contributing separator
docs: fix missing submodules docs

plugins/languages/vim-slime.nix

@@ -53,14 +53,14 @@ in
 
           Examples:
             - `tmux`:
-              ```
+              ```nix
                 {
                   socket_name = "default";
                   target_pane = "{last}";
                 }
               ```
             - `zellij`:
-              ```
+              ```nix
                 {
                   session_id = "current";
                   relative_pane = "right";