[DOP-19924] Add custom openapi middleware

[IlyasDevelopment]

Change Summary

Implemented middleware for custom OpenAPI & ReDoc support, updated the documentation.

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

Attention: Patch coverage is 67.96117% with 33 lines in your changes missing coverage. Please review.

Project coverage is 87.12%. Comparing base (a21908a) to head (4526be1).
Report is 80 commits behind head on develop.

Files with missing lines	Patch %	Lines
syncmaster/backend/middlewares/openapi.py	45.09%	25 Missing and 3 partials ⚠️
syncmaster/settings/server/static_files.py	58.33%	5 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #122      +/-   ##
===========================================
- Coverage    87.75%   87.12%   -0.63%     
===========================================
  Files          136      140       +4     
  Lines         3137     3240     +103     
  Branches       242      251       +9     
===========================================
+ Hits          2753     2823      +70     
- Misses         325      355      +30     
- Partials        59       62       +3     

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