ci: modernize documentation workflow

[whg517]

Summary

• replace the legacy Super-Linter setup with focused full-repository Markdown linting
• pin current Actions to immutable commits and use the built-in pip cache from setup-python
• run strict MkDocs checks on branch pushes and pull requests, then deploy only after checks pass on main
• scope repository write permission to the deployment job and serialize gh-pages updates
• convert the one existing bare URL to an autolink so full linting passes

Validation

• markdownlint-cli2 0.23.2: 30 Markdown files, 0 issues
• MkDocs strict build in a clean Python 3.12 environment
• actionlint 1.7.12
• git diff --check

Scope

requirements.txt remains unchanged so the existing dependency update in #43 stays independent.