docs(user-guide): document the invoker role

[s5unanow]

Summary

Documents the new Invoker role (EPMCDME-10371): a run-only, least-privileged user who can chat with shared assistants, run shared workflows, clone Marketplace assistants, and manage personal integrations, but cannot view or change any AI configuration. Covers what the role can and cannot do, how the reduced interface looks, how the role is assigned in Users Management, and how it compares with the other platform roles.

Changes

• Added docs/user-guide/project-user-management/invoker-role.md — full Invoker reference (permissions, restrictions, reduced UI, Marketplace clone flow, personal integrations, assignment rules, audit trail, role comparison) and registered it in sidebars.ts
• Updated docs/admin/security/roles-rbac.md — Invoker added to the role list, capability table, hierarchy diagram, plus an assignment/exclusivity/prerequisite section
• Updated docs/user-guide/project-user-management/users.md — "Assign the Invoker Role" steps under User Details and the new Auditor/Invoker values in the Platform Role filter
• Updated docs/user-guide/project-user-management/index.mdx (Invoker column in "Access by Role", FeatureCard) and docs/user-guide/getting-started/glossary.md (Invoker entry)
• Added two FAQ entries: faq/what-can-an-invoker-do-in-codemie.md, faq/how-do-i-assign-the-invoker-role-to-a-user.md

Testing

• Tested locally with npm run build (full production build; npm start was not used)
• All pages render correctly (verified in the built output)
• Images display properly (no new images in this PR)
• Internal links work (build runs with onBrokenLinks: 'throw')
• Sidebar navigation works (new entry under Project & User Management)

Quality Checks

• npm run check passes (typecheck + lint + commitlint)
• No MDX compilation errors
• No raw angle brackets (<text> must be `<text>`)
• 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

• Content is derived from the shipped behaviour of the backend (codemie MR !4072) and UI (codemie-ui MR !1773) implementations of EPMCDME-10371; it should be merged together with, or after, those changes reach a release.
• Text-only for now: the Users Management UI (enterprise edition) was not available in the local stack, so no new screenshots are included.
• The roles overview still does not describe the Auditor role; that is a separate gap and is intentionally left out of this PR.