Remove Git configuration sections from the developer documentation

Assuming this section only deals with the OMERO installation from source,
the Git configuration for contributing is irrelevant here and is also a
duplicate of the Contributing documentation.
Add placeholder for OMERO build dependencies
Add link to contributing documentation
Re-organize paragraphs
Typo fixes
Tidying up location of git info
Capital G in Git & other tidying
Add source code downloads paragraph and add link to contributing for Git
Fix the environment variable name
Add note about current caveat of the OMERO source code zip
merge develop -Dorg -Ssuccess-only: PR 953 (tweak server docs (rebased from dev_5_0))
merge develop -Dorg -Ssuccess-only: PR 954 (OMERO developer setup )
merge develop -Dorg -Ssuccess-only: PR 955 (Tidying up location of git info)
merge develop -Dorg -Ssuccess-only

Repository: openmicroscopy/ome-documentation
Already up-to-date.

Merged PRs:
  # PR 953 mtbc 'tweak server docs (rebased from dev_5_0)'
  # PR 954 sbesson 'OMERO developer setup '
  # PR 955 hflynn 'Tidying up location of git info'

Generated by OME-5.1-merge-push#413 (http://ci.openmicroscopy.org/job/OME-5.1-merge-push/413/)