Add documentation for server package

[ksysoev]

This pull request adds documentation for the server package. It includes descriptions for the core concepts, such as channels, dispatchers, and connections. It also provides information about the backend abstraction and request abstraction. Additionally, it adds documentation for middleware concepts and their usage. This documentation will help developers understand and use the server package more effectively.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 91.69%. Comparing base (50469b1) to head (05a8cc3).

Additional details and impacted files

@@           Coverage Diff           @@
##             main      #86   +/-   ##
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  Coverage   91.69%   91.69%           
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  Files          21       21           
  Lines         855      855           
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  Hits          784      784           
  Misses         52       52           
  Partials       19       19           

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