[clang-doc] Improving Markdown Output

This change has two components. The moves the generated file
for a namespace to the directory named after the namespace in
a file named 'index.<format>'. This greatly improves the browsing
experience since the index page is shown by default for a directory.

The second improves the markdown output by adding the links to the
referenced pages for children objects and the link back to the source
code.

Patch By: Clayton

Differential Revision: https://reviews.llvm.org/D72954