Unify contents of contributing section into a single page

- 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.

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

Bugs closed: NEST-33

Reviewed at https://reviews.imfreedom.org/r/789/