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/toggleterm.nix

@@ -19,7 +19,7 @@ in {
         Size of the terminal.
         `size` can be a number or function
         Example:
-        ```
+        ```nix
         size = 20
         ```
         OR
@@ -78,7 +78,7 @@ in {
       Highlights which map to a highlight group name and a table of it's values.
 
       Example:
-      ```
+      ```nix
         highlights = {
           Normal = {
             guibg = "<VALUE-HERE>";