feat(module:date-picker): add nzShowNow

[C-racker]

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
[x] 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: #6146

What is the new behavior?

Does this PR introduce a breaking change?

[ ] Yes
[x] No

Other information

[zorro-bot]

This preview will be available after the AzureCI is passed.

[codecov]

Codecov Report

Merging #6160 (08a28f4) into master (13f2281) will increase coverage by 0.05%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #6160      +/-   ##
==========================================
+ Coverage   89.81%   89.87%   +0.05%     
==========================================
  Files         468      468              
  Lines       14258    14257       -1     
  Branches     2243     2242       -1     
==========================================
+ Hits        12806    12813       +7     
+ Misses        896      887       -9     
- Partials      556      557       +1     

Impacted Files	Coverage Δ
...mponents/date-picker/date-range-popup.component.ts	91.86% <ø> (ø)
...omponents/date-picker/calendar-footer.component.ts	95.83% <100.00%> (+0.18%)	⬆️
components/date-picker/date-picker.component.ts	92.61% <100.00%> (+0.04%)	⬆️
components/table/src/table-style.service.ts	94.82% <0.00%> (-1.73%)	⬇️
components/upload/upload-list.component.ts	95.83% <0.00%> (-0.14%)	⬇️
components/select/select.component.ts	87.62% <0.00%> (-0.13%)	⬇️
components/modal/utils.ts	100.00% <0.00%> (ø)
components/modal/modal-types.ts	100.00% <0.00%> (ø)
components/button/button.component.ts	100.00% <0.00%> (ø)
components/modal/modal-footer.component.ts	100.00% <0.00%> (ø)
... and 4 more

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[wenqi73]

测试用例加一个

[wenqi73]

Thanks for your work! @C-racker