ci: deploy docs to GitHub Pages on push

[setupelz]

Summary

• Add .github/workflows/docs.yml: builds docs with mkdocs and publishes to gh-pages on push to main (paths: docs/, mkdocs.yml, src/, README.md), plus manual workflow_dispatch.

Lint & Test Results

• Lint: pre-existing ruff check . findings on main (118, unrelated to this change) - this PR's diff is limited to the new workflow file, no Python touched.
• Tests: not run - no code changes in scope.

Test Plan

• After merge, run gh workflow run docs.yml and confirm the run publishes to gh-pages.
• Verify https://setupelz.com/fair-shares/api/python-api/ returns 200 after the Pages CDN updates.