Document website pull request target

[JackieTien97]

Summary

• add a Website Contributions section to the root README
• state that website pull requests must use docs/dev as the base branch

Motivation

The repository README did not tell contributors which branch receives website changes.

Impact

Contributors can identify the correct base branch before opening website documentation pull requests.

Root cause

The website contribution workflow was not documented in the repository README.

Validation

• git diff --check

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 60.52%. Comparing base (46cf0a1) to head (fa52fad).

Additional details and impacted files

@@           Coverage Diff            @@
##           develop     #887   +/-   ##
========================================
  Coverage    60.51%   60.52%           
========================================
  Files          744      744           
  Lines        49081    49081           
  Branches      7797     7797           
========================================
+ Hits         29703    29705    +2     
+ Misses       17978    17975    -3     
- Partials      1400     1401    +1     

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