Support index.md auto-discovery in repo namespace

[pedro-psb]

What this does

• Remove magical behavior of popping "quickstart.md" from "tutorials" section and adding to the root repository namespace.
  • It was a very counter-intuitive behavior for achieving this "root" level page.
• Support explicit usage of index.md located in {repo}/{persona} to be used as root overview.
  • Although its a special-case treatment, it's much less surprising than the previous one.

Example:

File structure

/pulp-docs/
dev/
index.md
reference/
markdown-cheatsheet.md
(...)

Developer Manual > Pulp-Docs (render)

RPM (...)
Deb (...)
(...)
Pulp Docs
Overview (index.md)
Reference
Markdown Cheatsheet