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

flake.nix

@@ -117,7 +117,7 @@
             };
           };
           packages = {
-            docs = pkgs-unfree.callPackage (import ./docs.nix) {
+            docs = pkgs-unfree.callPackage (import ./docs) {
               modules = nixvimModules;
             };
             runUpdates =