Issue #3614886: Publish a documentation site via GitLab Pages (MkDocs)
Adds mkdocs.yml and splits the wiki content into per-topic pages under docs/ (14 pages: installation, permissions, configuration, both dashboards, usage log, rollup/retention, personal usage, GDPR, Drush, context grouping, hooks API, multi-currency). Uses the default gitlab_templates pages job, no CI changes. Verified locally with mkdocs build --strict (mkdocs-material, same as CI): builds clean, no broken links. Also removes the docs/ .gitignore entry and adds mkdocs/SSRF to the project cspell words.
Closes #3614886