docs: update Python version badges/text to 3.10+ (follows PR #31)

[brgsk]

Summary

• Update docs to reflect Python 3.10+ support.
• Aligns badges/text with packaging and CI from PR feat: relax Python version requirement #31.
• No functional code changes.

Changes

• README.md: Update Python version badge/alt to 3.10+.
• docs/index.md: Update Python version badge/alt to 3.10+.
• docs/installation.md: Update requirement text to “Python 3.10+”.

Related

• Follows up on PR feat: relax Python version requirement #31 (relax Python version requirement).

Checklist

• Tests pass (make all) — docs-only; no code changes
• Docs updated (if user-facing)

[claude]

This PR documents Python 3.10+ support, but PR #31 (which would relax the requirement) is still open and unmerged. The current pyproject.toml still has requires-python = ">=3.13" and CI only runs on Python 3.13.

Merging this would make the docs incorrect — users would see "Python 3.10+" but pip install would fail on 3.10/3.11/3.12.

This PR should only be merged after PR #31 lands. Until then, the badges and installation text should stay at 3.13+.