feat: doc quick insert placeholder

[siam-ese]

close #xxx

Pull Request Checklist

• Related tickets or issues have been linked in the PR description (or missing issue).
• Naming convention is followed (do please check it especially when you created new plugins, commands and resources).
• Unit tests have been added for the changes (if applicable).
• Breaking changes have been documented (or no breaking changes introduced in this PR).

[github-actions]

View Deployment

📑 Demo (React@19)	Demo (React@16)	📚 Storybook
🔗 Preview link	🔗 Preview link	🔗 Preview link

[github-actions]

Playwright test results

 25 passed

Details

 25 tests across 12 suites
 6 minutes, 5 seconds
 eb13c62
 For more information, see full report

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 35 lines in your changes missing coverage. Please review.

Project coverage is 34.12%. Comparing base (0477211) to head (eb13c62).

Files with missing lines	Patch %	Lines
...-ui/src/services/doc-quick-insert-popup.service.ts	0.00%	23 Missing ⚠️
...ck-insert-ui/src/views/KeywordInputPlaceholder.tsx	0.00%	4 Missing ⚠️
...ick-insert-ui/src/views/QuickInsertPlaceholder.tsx	0.00%	3 Missing ⚠️
...ocs-quick-insert-ui/src/views/QuickInsertPopup.tsx	0.00%	2 Missing ⚠️
.../docs-ui/src/services/doc-popup-manager.service.ts	0.00%	2 Missing ⚠️
.../src/controllers/doc-quick-insert-ui.controller.ts	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##              dev    #4841      +/-   ##
==========================================
- Coverage   34.14%   34.12%   -0.03%     
==========================================
  Files        2621     2623       +2     
  Lines      136845   136875      +30     
  Branches    30391    30394       +3     
==========================================
- Hits        46721    46703      -18     
- Misses      90124    90172      +48     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.
• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.