[RFC] Documentation/report overview page

The first commit of this PR was just to update the gem dependencies; `bundle install` complained otherwise:

~~~
Fetching source index from https://rubygems.org/
Could not find blankslate-2.1.3 in any of the sources
~~~

The page's URL will be neovim.org/doc, which is consistent with the URLs of the reports (neovim.org/doc/dev, ...).
I am not a native speaker; the text is merely a suggestion :-)
Name this "Clang Report" to maintain parallel form with "Coverity Report".