docs(user-guide): add auditor role documentation

[mykolanehrych]

Summary

Documents the new is_auditor boolean flag introduced in EPMCDME-10930. Covers the full feature across both admin and user-facing documentation.

Changes

• docs/admin/security/roles-rbac.md — added Auditor column to capability matrix, new "View all users/projects/budgets" row, updated role hierarchy diagram, and new ## Auditor Flag section with access table, combination rules, and assignment steps
• docs/user-guide/project-user-management/users.md — updated access admonition; added Platform Roles block description and ### Platform Roles assignment section with screenshots (toggle on/disabled+tooltip/auditor read-only view)
• docs/user-guide/project-user-management/projects.md — added Auditor read-only visibility note and screenshot to intro
• docs/user-guide/analytics/index.md — added ### Auditor access subsection; updated Personal Spending and Leaderboard blurbs
• docs/user-guide/budget-management/index.md — updated access text and table (Auditor column added, redirect note)
• faq/ — two new FAQ entries: assigning the Auditor flag, and what Auditors can see and do

Testing

• Tested locally with npm start
• All pages render correctly
• Images display properly
• Internal links work
• Sidebar navigation works

Quality Checks

• npm run check passes (typecheck + lint + commitlint)
• npm run build passes — no broken links
• No MDX compilation errors
• No raw angle brackets
• Sidebar references document IDs (not filenames)
• Images stored locally next to content (not in static/img/)
• Commit messages follow Conventional Commits
• No secrets or credentials in documentation

Additional Notes

Pre-commit secrets scan skipped locally (ghcr.io unreachable from Rancher Desktop at commit time; Gitleaks image has since been pulled). CI pipeline runs its own Gitleaks scan before merge.