Add skill to generate API markdown file

[JennyPng]

closes Azure/azure-sdk-tools#14656

• edited MAX_PYTHON_VERSION -> PYTHON_VERSION_LIMIT in apistub.py, bc the name was a bit misleading and was confusing Copilot - apistub supports Python versions strictly below PYTHON_VERSION_LIMIT

[Copilot]

Pull request overview

Adds a new GitHub “skill” doc for generating API markdown via azpysdk apistub --md, and clarifies the Python version gate in apistub.py to enforce “Python < 3.12” (since apistub errors on 3.12+).

Changes:

• Introduced .github/skills/generate-api-markdown/SKILL.md with steps to generate API markdown output.
• Updated apistub.py to use a Python version limit tuple (3, 12) and a clearer error message for unsupported versions.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 2 comments.

File	Description
eng/tools/azure-sdk-tools/azpysdk/apistub.py	Replaces “max version” logic with a “version limit (< 3.12)” guard and updates messaging.
.github/skills/generate-api-markdown/SKILL.md	Adds a skill guide documenting setup and the command to generate API markdown.

[mccoyp]

LGTM in terms of skill placement and naming