Migrate from Furo to unified PyData theme

[scriptautomate-bc]

Summary

Adds and expands on the pydata theme currently used on the Salt User Guide and Salt v3008 reference docs:

• Collapsible left-hand TOC nav items
  • Further optimizes base theme vs. base theme from salt-user-guide edits (need to port back to user guide and salt ref docs later)
• Better formatted/spaced left-hand TOC nav items
• Instead of "Table of Contents", will be the name of the specific docset (ex. "Salt Install Guide")
• Top nav will have a color change meaning "Active" so that you can tell which docset you are on (ex. "Install Guide" highlighted because on those docs)
• Updates devcontainer to Python 3.14 (non-Bitnami)

Related

• Fixes Migrate Sphinx theme from Furo to pydata #76
• Related: [DOCS] Salt Reference Docs: Sphinx Theme Refresh (Furo, Sphinx Book, or Alternative Third-Party Managed Theme) and Deprecation of saltstack2 custom theme salt#67090
  • Builds on work from:
    • Migrate documentation to PyData Sphinx Theme salt#69185
    • Move to pydata theme for unification salt-user-guide#10