Add infrastructure to build the contributing documentation set

This PR:
- adds the necessary infrastructure to build the Contributing documentation set
- adds `.gitignore` so that the build output is ignore by Git
- moves all images specific to the contributing documentation set to the `contributing/images/` folder
- creates a dummy `/contributing/_stati/` folder
- moves the definition of `omerocmd` to `common/conf.py` as it is used in this documentation set
- fixes the usage of `:doc:` vs `:omero_doc:` in various places of the contributing section

With this PR, the contributing section should be build able as usual using `make clean html`. All changes have been made so that the latter command completes without warning. I did not run any link checker at this point so some links may be invalid but I figured out you may completely rewrite them anyways.

Once this PR is merged and a PR is opened against the mother repository, we can set up the necessary job/Apache config to deploy the staging build
/cc @qidane, @joshmoore, @jburel