Add MkDocs Material to Python dependencies

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Motivation

• The site uses theme: material in mkdocs.yml and the Pages workflow failed with "Unrecognised theme name: 'material'", so explicit docs dependencies are required to ensure the CI job installs mkdocs-material.

Description

• Add requirements.txt with mkdocs>=1.6 and mkdocs-material>=9.5 so the workflow step python -m pip install -r requirements.txt will install the required documentation toolchain.

Testing

• Ran python -m pip install -r requirements.txt, which failed in this environment due to proxy/network restrictions when contacting the package index.
• Ran mkdocs build --strict --clean --site-dir dist, which failed locally because mkdocs is not available after the blocked dependency install.

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Codex Task