[Docs] Limit nesting of generated TOCs

The jekyll tocify plugin generates an HTML TOC based on the headings
of the markdown files. This commit limits the nesting of the TOC to 2,
which means that only h1 and h2 element will be taken into account.
Previously, all heading elements were used, which resulted in a very
cluttered TOC.

docs/_plugins/tocify.rb

 module Jekyll
   module Tocify
     def tocify(input)
-      converter = Redcarpet::Markdown.new(Redcarpet::Render::HTML_TOC)
+      converter = Redcarpet::Markdown.new(Redcarpet::Render::HTML_TOC.new(nesting_level: 2))
       converter.render(input)
     end
   end