docs: add ng-ant-admin website URL (#7213)

[huajian123]

PR Checklist

Please check if your PR fulfills the following requirements:

• 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)
• 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
[ ] 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
[ ] No

Other information

[zorro-bot]

This preview will be available after the AzureCI is passed.

[codecov]

Codecov Report

Merging #7230 (19e79a8) into master (662648c) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #7230   +/-   ##
=======================================
  Coverage   91.72%   91.72%           
=======================================
  Files         486      486           
  Lines       15918    15918           
  Branches     2588     2588           
=======================================
  Hits        14601    14601           
  Misses       1004     1004           
  Partials      313      313           

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