Add Sphinx directive to automate QPY version history table generation

[jathin-sr]

Fixes #16086

This PR introduces a Sphinx extension that dynamically generates the QPY version history table from git tags at documentation build time.

This removes the need to manually update the table during releases, which has previously led to missed updates (e.g. 2.3.1, 2.4.0).

Implementation details:

• Adds a custom directive in docs/_ext/
• Extracts QPY_VERSION and QPY_COMPATIBILITY_VERSION from tagged releases
• Handles legacy versions (< 0.20.0) manually
• Integrates into qiskit.qpy docstring via directive

Inspired by the versionutils.py extension used in the qiskit-metapackage repo.

AI/LLM disclosure

• I didn't use LLM tooling, or only used it privately.
• I used the following tool to help write this PR description: ChatGPT
• I used the following tool to generate or modify code: ChatGPT

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