Add doc layout improvements
Add simpler markup for language switcher

Had originally planned on building a widget for inline switching for
each separate example, but realised that the common use case will be to
switch language once and leave it at that. So there is no need to group
the code examples into "same semantics" lists.
Add frontmatter to all doc files
Fix internal links

I am not entirely happy with the relative links as they are now.
Relative links were brittle already with the use of GitHub's section
links, where links would break with changing method signatures. This
situation has now become even worse, where unrelated changes, e.g. an
additional method being added to the docs, can break links.

This is because I use RedCarpet's feature of automatically creating
heading identifiers and it doesn't use the text of the heading to do so.
Maybe going for HTML with IDs here is actually the most reliable choice.
Add language switching for inline code
Add sidebar