docs(site): enable syntax highlighting, code copy button, and light/dark palette

[CooperBigFoot]

docs(site): enable syntax highlighting, code copy button, and light/dark palette

What changed

• Extended the Material theme config with the content.code.copy feature and the two-entry light/dark palette toggle.
• Added the requested markdown_extensions block for pymdownx.highlight, pymdownx.inlinehilite, and pymdownx.superfences.

Why

The docs site previously rendered code blocks without syntax highlighting, copy buttons, or a theme toggle. pymdown-extensions was already installed in the docs environment, so no dependency changed.

Verification

• Gate 1: /Users/nicolaslazaro/Desktop/work/shed/crates/python/.venv/bin/python -c "import pymdownx.highlight, pymdownx.inlinehilite, pymdownx.superfences; print('pymdownx ok')" printed pymdownx ok and exited 0.
• Gate 2: config-presence grep command printed CONFIG-OK and exited 0.
• Gate 3: exclude_docs diff and comment grep printed EXCLUDE-OK and exited 0.
• Gate 4: strict MkDocs build command exited 0 and removed .mkdocs-site-check.
• Gate 5: rendered-output spot check printed RENDER-OK, exited 0, and removed .mkdocs-spot-check.
• Gate 6: added-line em dash check printed NO-EMDASH-OK and exited 0.
• Gate 7: diff-scope check printed SCOPE-OK and exited 0.