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/utils/coverage.nix

@@ -244,7 +244,7 @@ in {
         See plugin documentation for language specific options.
 
         Example:
-        ```
+        ```nix
           {
             python = {
               coverage_file = ".coverage";