Add merge instructions to release instructions

[zklaus]

Description

Closes #2133

Link to documentation: https://esmvaltool--2131.org.readthedocs.build/projects/ESMValCore/en/2131/contributing.html#mark-the-release-in-the-main-branch

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[codecov]

Codecov Report

Merging #2131 (39e6f5d) into main (a6639a7) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main    #2131   +/-   ##
=======================================
  Coverage   93.09%   93.09%           
=======================================
  Files         237      237           
  Lines       12795    12795           
=======================================
  Hits        11911    11911           
  Misses        884      884           

[remi-kazeroni]

Thanks a lot, @zklaus! That is nicely written and explained 👍

Once that is merged, we should add the same documentation for the Tool here.

@schlunma, @valeriupredoi - could one of you do a final check and merge, please?

[valeriupredoi]

Very nice indeed, cheers, chaps 🍺