docs(user-guide): add conversation finishing api guide

[bohdanmaliar]

Summary

Documents the shipped conversation finishing APIs so applications and schedulers can close chats that should no longer accept new messages.

Changes

• Added the Finish Conversations API page (owner finish, admin bulk finish, admin list filters, finished_at on reads, HTTP 409)
• Linked the page from the API index, Assistants hub, delete-chats note, glossary, and admin retention FAQ
• Added FAQ entries for finishing a conversation and whether auto-close after N hours exists

Testing

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

Verified with npm run check and npm run build (broken-link throw). No new images.

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

Ticket: EPMCDME-11067. There is no CodeMie UI control for finishing a chat; this is a platform API capability. Auto-expiry is not built in — an external scheduler can list unfinished conversations and call bulk finish.

[github-actions]

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