Merge changes from topic "sphinx-doc"

* changes:
  doc: Update link formats
  doc: Update page content after re-arrangement
  doc: Sync common content with TF-A
  doc: Move content from Porting Guide
  doc: Migrate content from User Guide and update it
  doc: Move tools info to Getting Started chapter
  doc: Move content to new About chapter
  doc: Copy readme content to docs/index.rst