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";

plugins/utils/leap.nix

@@ -60,7 +60,7 @@ in {
           either be defined as strings or tables.
 
           Example:
-          ```
+          ```nix
             [
               "\r\n"
               ")]}>"

plugins/utils/neogen.nix

@@ -93,7 +93,7 @@ in {
           https://github.com/danymat/neogen/blob/main/docs/adding-languages.md#default-generator
 
         Example:
-        ```
+        ```nix
           {
             csharp = {
               template = {

plugins/utils/presence-nvim.nix

@@ -84,7 +84,7 @@ in {
           `source`: The source of the asset, either an art asset key or the URL of an image asset.
 
           Example:
-          ```
+          ```nix
             {
               # Use art assets uploaded in Discord application for the configured client id
               js = [ "JavaScript" "javascript" ];

plugins/utils/todo-comments.nix

@@ -55,7 +55,7 @@ in {
             Configurations for keywords to be recognized as todo comments.
 
             Default:
-            ```
+            ```nix
             {
               FIX = {
                 icon = " "; # Icon used for the sign, and in search results.
@@ -144,7 +144,7 @@ in {
             of highlight groups or use the hex color if hl not found as a fallback.
 
             Default:
-            ```
+            ```nix
             {
               error = [ "DiagnosticError" "ErrorMsg" "#DC2626" ];
               warning = [ "DiagnosticWarn" "WarningMsg" "#FBBF24" ];
@@ -163,7 +163,7 @@ in {
             Arguments to use for the search command in list form.
 
             Default:
-            ```
+            ```nix
             [
               "--color=never"
               "--no-heading"

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>";