Add some basic content to the index of the development section so that

  it's not empty
Remove contributing/mercurial page since its contents are now in the docs

  page for using mercurial or in the (now single) contributing page.
Add section in contributing page that points to useful link on how to

  contribute code.
Minor cleanups in the pre-existing contributing page
Fix broken link in about index page that appeared after moving the

  contributing section into a single page.

Ran dev-server.sh and verified contents of the website looked as intended