Make the Getting Started section more intuitive

[patiencedaur]

Epics:
#2051 – Restructuring Tarantool docs
#2052 – Integrate docs and trainings

To do

Let's consolidate Getting Started pages in a unified Getting Started section. The audience of Getting Started is beginners who don't yet know much about Cartridge, app server functionality, etc. So it seems better to consolidate the Getting Started pages than to attach a separate Getting Started guide to every section.

Feedback

1. @ igormunkin:
   "Слушайте, а помните полгода назад еще был нормальный Getting Started без Cartridge: а он куда-то делся или я ослеп?"
   "А вот тот, где там box.insert с группами был, его видимо уже не найти?"
   "Нашёл! Но было сложно."
   https://www.tarantool.io/en/doc/latest/getting_started/getting_started_db/#creating-db-locally

2. This page (Pokemon Go guide) is apart from the other Getting Started pages:
   https://www.tarantool.io/en/doc/latest/book/app_server/creating_app/

3. See this discussion: Feedback about Quick Start #2810