feat(module:dropdown): allow backdrop to be disabled

[lppedd]

Needs someone that translate the documentation to Chinese, I'm afraid I cannot 😄
See "current behavior" for explanation.

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
[ ] Documentation content changes
[ ] Application (the showcase website) / infrastructure changes
[ ] Other... Please describe:

What is the current behavior?

Currently the backdrop is enforced when using trigger = 'click'.
This causes usability problems when used in conjunction, for example, with a custom table filter.

What is the new behavior?

The backdrop can be disabled by setting nzBackdrop = false.

Does this PR introduce a breaking change?

[ ] Yes
[X] No

Other information

[netlify]

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Built with commit 5d106ca

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[codecov]

Codecov Report

Merging #3769 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #3769      +/-   ##
==========================================
+ Coverage   95.38%   95.38%   +<.01%     
==========================================
  Files         706      706              
  Lines       14506    14508       +2     
  Branches     1913     1914       +1     
==========================================
+ Hits        13836    13839       +3     
  Misses        243      243              
+ Partials      427      426       -1

Impacted Files	Coverage Δ
components/dropdown/nz-dropdown.directive.ts	89.09% <100%> (+0.13%)	⬆️
components/select/nz-option-container.component.ts	97.67% <0%> (+2.32%)	⬆️

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[vthinkxie]

Thanks for your PR

[lppedd]

@vthinkxie my pleasure. Fixed documentation. Thanks!