docs(legend): 图例完善默认不选中以及点击选中交互的文档 & 增加 demo

[visiky]

Checklist

• npm test passes
• tests and/or benchmarks are included
• commit message follows commit guidelines

Description of change

[coveralls]

Pull Request Test Coverage Report for Build 694470602

Warning: This coverage report may be inaccurate.

We've detected an issue with your CI configuration that might affect the accuracy of this pull request's coverage report.
To ensure accuracy in future PRs, please see these guidelines.
A quick fix for this PR: rebase it; your next report should be accurate.

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 7 unchanged lines in 1 file lost coverage.
• Overall coverage decreased (-0.07%) to 93.522%

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Files with Coverage Reduction	New Missed Lines	%
src/chart/controller/gesture.ts	7	82.14%

Totals
Change from base Build 685937074:	-0.07%
Covered Lines:	10904
Relevant Lines:	11307

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💛 - Coveralls

[hustcc]

这个的答疑量很高！以后就可以甩文档了。