Remove plonematch theme from OME documentation

Sphinx theme for the HTML documentation has been moved to a standalone repository https://github.com/openmicroscopy/sphinx_theme and is used to generate documentation in the CI builds. This PR:

- removes plonematch theme from the repository
- fixes the environment variables description in the Readme
- updates the description of the docs job in the continuous integration documentation page
- fixes the Travis build

To test this PR:
- check the OMERO and FORMATS merge builds are still passing and the menu is correctly rendered on the staging pages
- check the changes of the Readme and the continous integration pages
- locally test the documentation HTML and PDF builds

:warning: Before merging this PR, make sure the release doc build configurations have been updated to use the new sphinx_theme repository