Merge pull request #408 from hflynn/11177

Adding What's new sections
Remove plonematch theme from ome-documentation repository

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.

Only impact of this commit on local builds is that "-D html_theme=plonematch"
can not be used in the SPHINXOPTS envvar anymore.
Remove mention of plonematch theme in the main README
Remove plonematch html_theme from Travis build envvar
Fix  configuration section of continuous integration documentation page
merge develop -D org -I include -E exclude: PR 369 (Cpp: Correct content and markup)
merge develop -D org -I include -E exclude: PR 404 (Updating Download screenshots for Version 5 (FS))
merge develop -D org -I include -E exclude: PR 412 (Remove plonematch theme from OME documentation (rebased onto develop))