docs: Add note on releasing a pre-release

[ivanvc]

Spawned from #19011 (review). Update the release documentation to specify how to do a pre-release from the main branch.

Fixes #19010.
Please read https://github.com/etcd-io/etcd/blob/main/CONTRIBUTING.md#contribution-flow.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 68.77%. Comparing base (c1171da) to head (5a378e7).
Report is 6 commits behind head on main.

Additional details and impacted files

see 25 files with indirect coverage changes

@@            Coverage Diff             @@
##             main   #19016      +/-   ##
==========================================
- Coverage   68.77%   68.77%   -0.01%     
==========================================
  Files         420      420              
  Lines       35623    35623              
==========================================
- Hits        24500    24498       -2     
- Misses       9694     9702       +8     
+ Partials     1429     1423       -6     

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[ivanvc]

/retest

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