docs: [MTG-1221] Add comprehensive architecture documentation (#420)

* docs: comprehensive documentation update

- Add detailed system architecture documentation with component descriptions and data flow

- Clean up and modernize README with clearer structure and links to architecture docs

- Update CONTRIBUTING guidelines with detailed setup instructions and best practices

The changes provide a complete overview of the system's architecture, components, data flow, and database structure, while making the documentation more accessible and maintainable.

* docs: Update README.md

Co-authored-by: Oleksandr Mykhailenko <[email protected]>

* chore: update docs and remove deprecated clippy config

- docs(CONTRIBUTING.md):
    - Remove explicit Rust version (using rust-toolchain.toml).
    - Add note about bootstrapping Postgres for local integration tests.
    - Clarify feature branch naming convention (ticket number optional).
    - Remove redundant unit test command from CI section.
    - Add merge strategy (squash merging).
    - Improve clarity and formatting.
- docs(architecture.md):
    - Remove specific class dependencies, listing high-level dependencies instead.
    - Clarify BigTable's role as a data origin for the Slots DB.
    - Remove `explorer_client` (uncommitted code).
    - Correct formatting and improve wording.
- chore: Remove deprecated and faulty .clippy.toml.

* docs: refine release process

* docs: fix api type

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Co-authored-by: Oleksandr Mykhailenko <[email protected]>