docs: update releasing.md to mention npm publish workflow, fix a link to release PR workflow

[pichlermarc]

Which problem is this PR solving?

I added another workflow recently #5067 - this PR:

• adds some instructions for that new workflow, and replaces the old ones
• adds some extra comments to explain the added workflow from ci: add workflow to publish packages to npm #5067, some things I figured out myself over time and some of the knowledge there was handed down to me a long time ago but never put into writing
• fixes a broken link for the workflow to create release PRs

How Has This Been Tested?

• we ran through the flow that's explained here with a subset of @open-telemetry/javascript-maintainers yesterday

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 93.18%. Comparing base (eb3ca4f) to head (b922e42).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #5087   +/-   ##
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  Coverage   93.18%   93.18%           
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  Files         315      315           
  Lines        8086     8086           
  Branches     1617     1617           
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  Hits         7535     7535           
  Misses        551      551