[MRG] New docs theme

[dvorakvaclav]

Fixes #2927.

For info, I don't know if it's only for me, but in order to generate the docs I had to remove one letter from line 1261 in gensim/utils.py: Yield chunks as np.ndarrays instead of lists?. Otherwise the doc generation process ended with the error gensim/utils.py:docstring of gensim.utils.chunkize:9:Inline interpreted text or phrase reference start-string without end-string. This change is not included in this PR – just the files from docs/src.

[gojomo]

With no opinion on any of the actual core content here, this seems like a lump of files that perhaps shouldn't be inside the open-source library source-control. The project previously had about ~850 files; this PR changes 2111 files. It includes 1300+ SVG files, 300+ JPGs, and 50+ PHP (?!?) files. But I'm not sure there's yet any actual docs (rather than just styling) in this grouping. Perhaps this belongs in some separate "website" git project?

[piskvorky]

I'm pretty sure most of the files are not needed. @dvorakvaclav can you trim this PR down please? Let's review & commit & maintain only what we actually need.

@gojomo the actual content is in the 11 HTML files (templates). Yes, this project is "just styling" – actual content is still generated dynamically from .rst files with Sphinx with make html, no changes there.

[dvorakvaclav]

We deleted over 2000 unnecessary files imported from original html theme including old sphinx gensim theme. Now there should be only files, that are actually used.

[piskvorky]

My review from Trello (sorry guys, Czech only):

1. Social ikony dole jsou zabite: facebook ikona vede na github, github ikona chybi, ta treti ikona nevim co je. Celkove je tato sekce nepekna, ikony jsou umistene divne, bez logiky stranky. Prosim "socializovat" nejak lepe.
2. Favicon lepe pozitivni verzi (bez tmaveho pozadi)?
3. U sekce Scalable statistical semantics Analyze plain-text documents for semantic structure Retrieve semantically similar documents jako navstevnik nevim, na co se divam. Sloupce nijak oddelene, hesla se k nicemu nevztahuji, ani u nich neni zadna ikona nebo obrazek.
   Pohledem na puvodni verzi toto byl "checklist" hlavnich benefitu Gensimu. Prosim body presunout nad sample kod (nebo vedle?), namisto tlacitka "Explore all features", ktere nepotrebujem, a nejak je lepe vizualizovat.
4. Kliknuti na odkazy ke stazeni (napr. Download Python source code: run_core_concepts.py nebo Download Jupyter notebook: run_core_concepts.ipynb v docs/auto_examples/core/run_core_concepts.html, a nejspis i jinde) by melo soubor stahnout. Ne otevrit v prohlizeci jako text.
5. Chrome mi ukazuje chybu pri stahovani jednoho souboru, transport.png: Access to image from origin 'null' has been blocked by CORS policy: Cross origin requests are only supported for protocol schemes: http, data, chrome, chrome-extension, https.
6. Poskakovani stranky pri (automatickem) posunovani v sekci Who is using Gensim? je velice neprijemne.
7. Jak budeme v budoucnu menit (pregenerovat) ten syntax-highlighted sample kod nahore? Jaky je zde postup.
8. Stahovat 3MB na jednoduche staticke strance je docela dost. Napada vas, jak page load optimalizovat / urychlit? (krome CDN, kde pouzivame CloudFlare)

Otherwise the build generation works smoothly, tested locally 👍 I have to remember to re-apply 0b12d9d when fixing the index page copy (a new commit in develop missing from this PR).

[piskvorky]

@dvorakvaclav I committed fixes & changes to the index page into https://github.com/RaRe-Technologies/gensim/tree/new_docs_theme. Please merge that into this PR first, before any further changes, to avoid git conflicts. Or give me push access to your gensim fork.

[piskvorky]

I rebased & force-pushed, to remove the extra 2k files from our repo git history.

[piskvorky]

@gojomo @mpenkov final version here: https://radimrehurek.com/gensim_redesign/

Have a look and let me know if you see any issues – otherwise I'll merge tomorrow, the redesign is complete! Great job by @dvorakvaclav in my opinion.