docs: add autofocus for overview page search-input

[hsuanxyz]

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]

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[codecov]

Codecov Report

Merging #5648 into master will decrease coverage by 0.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #5648      +/-   ##
==========================================
- Coverage   90.31%   90.29%   -0.02%     
==========================================
  Files         455      455              
  Lines       13788    13788              
  Branches     2258     2258              
==========================================
- Hits        12452    12450       -2     
- Misses        819      820       +1     
- Partials      517      518       +1     

Impacted Files	Coverage Δ
components/core/util/text-measure.ts	89.56% <0.00%> (-1.74%)	⬇️

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