Move 'contributing' to top level of docs

[hflynn]

The 'Contributing to OMERO' section is actually contributing to OME and should be moved out of the OMERO docs and to the top level so it is clearly signposted to developers who want to be more involved.

Need to decide how best to do this as it doesn't need to be versioned for 4.4 or 5.

[hflynn]

Shunting out of testing - this needs some further thought before we rush into adding Jenkins builds and messing with the menus and top level docs structure again. I suggest we revisit after we've sorted out the new user guide location and made a firm decision on the future of the repo.

[hflynn]

Plan is to have this on the develop branch only at this stage. Have started work on a local branch and will get a build set-up with @sbesson

[hflynn]

Jenkins builds set up, docs deployed at http://www.openmicroscopy.org/site/support/contributing-staging/ and http://www.openmicroscopy.org/site/support/contributing/

[hflynn]

Initial move now merged - ​https://github.com/openmicroscopy/ome-documentation/pull/553 and PR is open to add it to the website menu ​https://github.com/openmicroscopy/sphinx_theme/pull/6
Other tickets are already in place to manage changes to the actual content so closing this.

Note: these docs have moved on develop branch only. Some changes to the using-git content, which is linked in scripting docs etc, may be back-ported to dev_4_4 but the docs will stay under OMERO developer docs on that branch.