website: remove mercurial-hosting.html

The instructions here applied to petsc.cs.iit.edu, which has been gone
for several months.  Now everything is on bitbucket to begin with and Hg
is no longer the primary SCM.
manual: fix typo in inequality box constraints
website: point users to 'maint' branch
website: replace 'hg bisect' with 'git bisect'
website: simpler Git installation instructions
docs: remove stale Mercurial reference
setup.py: fix link to master.tar.gz
website: update instructions for downloading petsc-dev
Merge branch 'jed/website-developers'

* jed/website-developers:
  website: update instructions for downloading petsc-dev
  setup.py: fix link to master.tar.gz
  docs: remove stale Mercurial reference
  website: simpler Git installation instructions
  website: replace 'hg bisect' with 'git bisect'
  website: point users to 'maint' branch
  website: remove mercurial-hosting.html