docs(web-components): update wc readme

[kennylam]

Update @carbon/web-components README.

Changelog

Changed

• update instructions and examples
• clean up markdown

Removed

• outdated build instructions

Testing / Reviewing

{{ Add descriptions, steps or a checklist for how reviewers can verify this PR works or not }}

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 84.34%. Comparing base (1d57a3a) to head (3cabdef).

Additional details and impacted files

@@           Coverage Diff           @@
##             main   #18716   +/-   ##
=======================================
  Coverage   84.34%   84.34%           
=======================================
  Files         408      408           
  Lines       14645    14645           
  Branches     4791     4771   -20     
=======================================
  Hits        12352    12352           
- Misses       2131     2132    +1     
+ Partials      162      161    -1     

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