[Documentation] Add additional design documentation

[mrLSD]

Description

Add a separate section with documentation, which should contain:

• Detailed design overview
• Overview of the library structure
• Structural overview of the basic components
• Documentation on the use of existing types, traits, and functions - in the form of a general overview
• Description of ways to interact with a data source in the form of AST
• Description of interaction with Codogen - as the main consumer of the results of the semantic analyzer