docs: define a standalone contribution process for executable Python recipes #562

Observed problem

AGENTS.md links to the central organization writing standard; cookbook rules differ from package repositories because examples, recipes, templates and published documentation must actually execute against supported dependencies and database versions.

Change

• Explain local issue/PR format, recipe setup, pinned requirements, smoke/golden-output validation, database availability and docs synchronization in AGENTS.md/CONTRIBUTING.md.
• Remove references that make the organization writing policy mandatory, while keeping useful links to real drivers and services that recipes demonstrate.
• Keep existing recipe-specific validation, offline checks, docs tests and real CUBRID 11.2/11.4 smoke coverage.
• Clarify that the cookbook is not governed by a generic Python wheel/PyPI release checklist.

Done when

• Each proposed recipe can be reviewed and reproduced from local instructions.
• The actual CI lanes and skip behavior are accurately described.
• No local guidance depends on rules housed in cubrid-lab/.github.
• No unrelated example or generated output is changed.