Update documentation to use Pillow instead of PIL (rebased onto develop)



This is the same as gh-653 but rebased onto develop.

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Related to https://github.com/openmicroscopy/openmicroscopy/pull/2077, this PR:

- replaces the top-level `Python Imaging Library` substitution by a `Pillow` substitution
- updates all occurrences of PIL by Pillow across the documentation
- adds a paragraph to the server upgrade section regarding Imaging Library for 5.0.0 and onwards