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

docs/SUMMARY.md

@@ -0,0 +1,11 @@
+# Structure for nixvim docs 
+
+# Options
+
+@NIXVIM_OPTIONS@
+
+#
+
+---
+
+[Contributing](./CONTRIBUTING.md)