Remove plonematch theme from Bio-Formats documentation (rebased onto develop)



This is the same as gh-593 but rebased onto develop.

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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 the plonematch theme from the `docs/sphinx` folder
- removes the deprecated `plone` target from the Sphinx Makefile

To test this PR:

- check the BIOFORMATS docs merge builds is still passing and the menu is correctly rendered on the staging pages
- check the HTML and PDF documentation sets still build locally

/cc @hflynn