Add dark mode, versioned docs, navigation tabs, and PR validation

[biplovbhandari]

Summary

• Add dark mode toggle (default/slate schemes).
• Add versioned docs via mike (version switcher dropdown, dev/latest on push to main, versioned deploy on tag push).
• Restructure nav into tabs: Home, Guide, API Reference, Notebooks.
• Add PR validation build (mkdocs build --strict) to catch errors before merge.
• Add edit page pencil icon, search autocomplete, content tooltips.
• Add extensions: attr_list, md_in_html, pymdownx.snippets, pymdownx.tasklist.
• Add mike to requirements-docs.txt.

Validation

• Local build and mike serve verified: tabs, dark mode, version switcher, edit icon all working.