[MRG] Migrate tutorials & how-tos to 4.0.0

[piskvorky]

Part of #2960.

[piskvorky]

@gojomo I went over our guides & tutorials, and all necessary migrations are in 502b654. It went pretty smoothly, I only wasn't sure about doc2vec (see the one FIXME in diff). Overall migration shouldn't be a major issue, I'm happy about that.

I did see some uses of build_vocab() in the tutorials. These might need another touch after your #2944 is merged (maybe), I'll keep that in mind.

[piskvorky]

Should be done now. I'll remove my internal test hacks & regenerate all docs next, so there will be lots of autogenerated auto_examples files in this PR soon.

My migration notes:

word2vec

• .vocab => .index_to_key
• size=100 => vector_size=100
• model.analogies() => model.wv.evaluate_word_analogies()
• index2word => .index_to_key

doc2vec

• .docvecs => .dv
• .index_to_doctag => .index_to_key
• dv.count = len(dv)

fasttext

• shenanigans around OOV / in-vocab for word in model ~ word in model.wv.key_to_index

[piskvorky]

Actually, before I push the autogenerated auto_examples – do you want to review too @mpenkov ? After the push, the PR will be a mess of files.

I already pushed auto_examples for the four core tutorials by accident – only the files in gallery matter for review.

Edit: The regenerated docs are now available at https://radimrehurek.com/gensim_redesign/
I'll wait for @mpenkov review before I push auto_examples & merge.

[piskvorky]

OK I'll merge, I need to continue with the other tickets. @mpenkov please let me know if you see anything wrong here whenever you get to it. I'll fix ex-post.