[clang-doc] Add second index for sections within info's content

This new index contains links to the main section of infos: Namespaces, Records, Functions, Enums, Members.
Also to each child function or enum.
Index is currently rendered on top of the info content, this will be fixed later with CSS.

Depends on D65690.

Differential Revision: https://reviews.llvm.org/D65030

llvm-svn: 368209