README: turn all extensions into headers for direct linking

Change all `**Extension: ...**` lines into headers to allow permalinking to a specific extension.
README: reduce 1:1 Topic -> Extension nesting

Reorganize some sections whose only purpose was containing a single Extension subsection.
README: blank line after headers
README: make all extension headers H4
Merge pull request #1461 from cben/readme-linkable-extensions

README: linkable extensions