docs: optimize mobile experience

[wzhudev]

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
[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

• Waiting for select and popover to fix bugs.
• More smooth transitions.
• Affix width.

[netlify]

Deploy preview for ng-zorro-master ready!

Built with commit 5fc29b2

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[codecov]

Codecov Report

Merging #2789 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #2789   +/-   ##
=======================================
  Coverage   95.52%   95.52%           
=======================================
  Files         517      517           
  Lines       12258    12258           
  Branches     1691     1691           
=======================================
  Hits        11710    11710           
  Misses        189      189           
  Partials      359      359

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[vthinkxie]

LGTM