Switch to default Sphinx theme

[Copilot]

Replaces the Read the Docs theme with Sphinx's default Alabaster theme for documentation.

Changes:

• Remove sphinx_rtd_theme import and dependency
• Set html_theme = 'alabaster' in docs/conf.py
• Remove sphinx_rtd_theme from tox docs environment dependencies
• Remove sphinx_rtd_theme from GitHub Actions workflow

Original prompt

Switch to the default sphinx theme

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[csett86]

@copilot Also check the github actions and remove the unused theme from there

[Copilot]

@copilot Also check the github actions and remove the unused theme from there

Done. Removed sphinx_rtd_theme from .github/workflows/tests.yml in commit 7d37b71.