DOC: progressbar_theme is undocumented

[st-bender]

Issue with current documentation:

In pymc.sample() and probably others, the option progressbar_theme is undocumented.

Idea or request for content:

Properly document the progressbar_theme option, with examples.
For example, I'd like to get the [sampled/total] display back as I find it useful to see in cases the sampler got stuck and proceeded only slowly. Only percentages is not very helpful in those cases. Any guide on how to get that would be appreciated.

Also, I could not find any previous issues, my apologies if that has been raised before.

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