[Tech debt] Integrate docs and trainings

[NickVolynkin]

Integrate Tarantool documentation and trainings:

• Write training instructions in rST/Sphinx, publish via the same pipeline to a closed part of tarantool.io
• Use the same page desing for docs and training instructions
• Translate slides & instructions via Crowdin
• Use verifiable links from trainings to docs
• Add training materials to search results for user with access to them

• Make the Getting Started section more intuitive #2805

[p7nov]

Closing: out of scope. Trainings have been moved to a separate platform.