feat(module:menu): support unfolds/folds & add demo and api docs

[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
[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: #206

What is the new behavior?

Does this PR introduce a breaking change?

[ ] Yes
[X] No

Other information

[coveralls]

Coverage decreased (-0.02%) to 40.204% when pulling 5a29c67 on HsuanXyz:feature-menu-collapsed into 74b2506 on NG-ZORRO:master.

[vthinkxie]

It would be better not change any style file, if must, please add custom style in patch.less file other than the index.less since it will be sync with ant design react.

[vthinkxie]

It would be better not change any style file, if must, please add custom style in patch.less file other than the index.less since it will be sync with ant design react.

[coveralls]

Coverage increased (+0.8%) to 40.973% when pulling 8224d0e on HsuanXyz:feature-menu-collapsed into 74b2506 on NG-ZORRO:master.