[DOP-21387] Rebuild documentation structure

[dolfinus]

Change Summary

• Added docs/quickstart section with instruction how run all Data.Rentgen components in just one command, and how to connect Spark & Airflow to Kafka.
• Added docs/entities page describing all entities in Data.Rentgen, and their relations.
• Renamed docs/backend to docs/reference. For each section, like docs/reference/server, docs/reference/consumer, move installation instruction to index.rst, and rewrite it a bit.
• Added docs/reference/database section describing how to run PostgreSQL (with and without docker). Moved here instruction how to run migrations and create partitions (previously this was described twice, in server and in consumer docs). Added note that partition creation script should be run on schedule.
• Added docs/reference/broker section describing how to run Kafka.
• Added frontend container to docker-compose.yml and docs/reference/frontend describing how to start and configure it.

It's a bit hard to read the diff, please see https://data-rentgen--108.org.readthedocs.build/en/108/

Related issue number

Checklist

• Commit message and PR title is comprehensive
• Keep the change as small as possible
• Unit and integration tests for the changes exist
• Tests pass on CI and coverage does not decrease
• Documentation reflects the changes where applicable
• docs/changelog/next_release/<pull request or issue id>.<change type>.rst file added describing change
  (see CONTRIBUTING.rst for details.)
• My PR is ready to review.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 92.36%. Comparing base (68a1ac6) to head (1c2c4ce).

Additional details and impacted files

@@           Coverage Diff            @@
##           develop     #108   +/-   ##
========================================
  Coverage    92.36%   92.36%           
========================================
  Files          154      154           
  Lines         3355     3355           
  Branches       227      227           
========================================
  Hits          3099     3099           
  Misses         199      199           
  Partials        57       57           

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