cleaned up extension development docs

- added a copyright header
- applied the same order of titles/headers used in other docs (typically
  === underline+overline for top level, === underline for second level,
  --- underline for third level, ~~~ underline for fourth level, etc)
- used ``double backticks`` around literals
- removed excessive whitespace
- replaced a plaintext link with an embedded link

Change-Id: Ic2bf8ed65433c82d0bdfb8dc2d8a18a0b62d6c30
Merge "cleaned up extension development docs"