doc: update feature development guide.

[siyuanfoundation]

Please read https://github.com/etcd-io/etcd/blob/main/CONTRIBUTING.md#contribution-flow.

Update the feature development process to reflect the changes brought by feature gates.

[siyuanfoundation]

/cc @ahrtr @ivanvc @jmhbnz @serathius

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 68.88%. Comparing base (040e5ed) to head (3a6b064).
Report is 10 commits behind head on main.

Additional details and impacted files

see 21 files with indirect coverage changes

@@            Coverage Diff             @@
##             main   #19625      +/-   ##
==========================================
+ Coverage   68.78%   68.88%   +0.10%     
==========================================
  Files         421      421              
  Lines       35897    35897              
==========================================
+ Hits        24692    24728      +36     
+ Misses       9783     9744      -39     
- Partials     1422     1425       +3     

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[ivanvc]

/retest

[siyuanfoundation]

/cc @fuweid

[k8s-ci-robot]

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