Upgrade MkDocs deployment toolchain

[boundless-forest]

Upgrade the documentation toolchain to current supported versions and align local development with deployment builds. This improves reproducibility, dependency validation, and CI performance while preserving the existing site behavior.

Main changes:

• Pin MkDocs, Material, and active plugins to verified versions and remove the unused committers plugin
• Standardize local and deployment builds on Python 3.12 with explicit dependency checks and strict MkDocs builds
• Upgrade checkout and Python setup actions to v7 and enable dependency caching across all deployment environments

Reviewer notes:

• mkdocs-redirects remains intentionally pinned to 1.2.2 because 1.2.3 adds ProperDocs without features needed by this site
• Validated in a clean Python 3.12 environment with pip check, strict site build, existing documentation verification scripts, and desktop and mobile visual checks

[github-actions]

• comment: aad9cdf
  preview: https://degov-docs-kbjvc28m3-itering.vercel.app