[IMPR] Blend documentation's layout with default layout

Adding a couple of improvements related to the documentation
and its methods, routes, style, etc.

• Adjusting method handling /docs/<doc> route
• Nesting Pycco-generated templates with default layout
• Including a minimalist sitemap in newlyc created doc.html template
• Cloning no-contributions.html into a generic 404.html
• Replaced strongly typed doc_list with a method, along with its test
• Related CSS customization

Bug: T203713
Change-Id: Ieee834b0c6523c5ecf12f0e1c7bfdb5d2aacfd6b