[Doc] Add English tutorial for Webcam API

[ly015]

Motivation

• Add an English tutorial for Webcam API
• Add README for hand gesture demo

Modification

BC-breaking (Optional)

Use cases (Optional)

Checklist

Before PR:

• I have read and followed the workflow indicated in the CONTRIBUTING.md to create this PR.
• Pre-commit or linting tools indicated in CONTRIBUTING.md are used to fix the potential lint issues.
• Bug fixes are covered by unit tests, the case that causes the bug should be added in the unit tests.
• New functionalities are covered by complete unit tests. If not, please add more unit tests to ensure correctness.
• The documentation has been modified accordingly, including docstring or example tutorials.

After PR:

• CLA has been signed and all committers have signed the CLA in this PR.

[codecov]

Codecov Report

Merging #1413 (7491241) into master (c8e91ff) will decrease coverage by 0.00%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #1413      +/-   ##
==========================================
- Coverage   84.35%   84.34%   -0.01%     
==========================================
  Files         232      232              
  Lines       19279    19279              
  Branches     3468     3468              
==========================================
- Hits        16262    16261       -1     
  Misses       2152     2152              
- Partials      865      866       +1     

Flag	Coverage Δ
unittests	84.28% <ø> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
mmpose/datasets/pipelines/shared_transform.py	83.26% <0.00%> (-0.43%)	⬇️

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