docs: add getting started guide for EEST opcode minilang

[MozirDmitriy]

• Added a new section to docs/writing_tests/writing_a_new_test.md with a small guide for using the Python-based EEST opcode minilang to write smart contracts in execution-spec-tests.
• Included examples for using the Opcodes class to generate bytecode, and described higher-level constructs such as Switch and Case for advanced control flow.
• Added references to the evm_bytes CLI tool for converting EVM bytecode to the Python minilang.
• Clearly stated that Yul source is no longer allowed in Python test cases (see PR refactor(fill,deps): forbid yul code in python tests; remove solc-select dependency #1779).
• Linked to relevant code and documentation sections for further reading.

Fixes #1809

[danceratopz]

@MozirDmitriy as-is, this is a drive-by PR that creates more work for us than if we'd done it ourselves. Can you read the issue, check the docs and put this somewhat in a relevant place?

[MozirDmitriy]

@MozirDmitriy as-is, this is a drive-by PR that creates more work for us than if we'd done it ourselves. Can you read the issue, check the docs and put this somewhat in a relevant place?

Yes, sorry, done.

[danceratopz]

Thanks. Please find a few comments below on how to link to the relevant data structures.

[MozirDmitriy]

Thanks. Please find a few comments below on how to link to the relevant data structures.

done

[danceratopz]

Thanks for the drive on this one @MozirDmitriy gg!