refactor(module:mention): migrate demo to standalone mode

[Laffery]

PR Checklist

Please check if your PR fulfills the following requirements:

• [x] The commit message follows our guidelines: https://github.com/NG-ZORRO/ng-zorro-antd/blob/master/CONTRIBUTING.md#commit
• [ ] Tests for the changes have been added (for bug fixes / features)
• [x] Docs have been added / updated (for bug fixes / features)

PR Type

What kind of change does this PR introduce?

• [ ] Bugfix
• [ ] Feature
• [ ] Code style update (formatting, local variables)
• [ ] Refactoring (no functional changes, no api changes)
• [ ] Build related changes
• [ ] CI related changes
• [x] Documentation content changes
• [ ] Application (the showcase website) / infrastructure changes
• [ ] Other... Please describe:

What is the current behavior?

Issue Number: N/A

What is the new behavior?

Does this PR introduce a breaking change?

• [ ] Yes
• [x] No

Other information

[zorro-bot[bot]]

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[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 93.23%. Comparing base (f9f7b9e) to head (03aef49). Report is 8 commits behind head on master.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #8787 +/- ## ========================================== + Coverage 93.17% 93.23% +0.06% ========================================== Files 538 537 -1 Lines 18457 18456 -1 Branches 2816 2816 ========================================== + Hits 17197 17208 +11 + Misses 1222 1210 -12 Partials 38 38 ``` | [Flag](https://app.codecov.io/gh/NG-ZORRO/ng-zorro-antd/pull/8787/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=NG-ZORRO) | Coverage Δ | | |---|---|---| | [](https://app.codecov.io/gh/NG-ZORRO/ng-zorro-antd/pull/8787/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=NG-ZORRO) | `93.23% <ø> (+0.06%)` | :arrow_up: | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=NG-ZORRO#carryforward-flags-in-the-pull-request-comment) to find out more.

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