docs(PUBLISHING): update publish back-merge instruction

[booc0mtaco]

Add a yarn install in the back-merge step, to account for edge case where next has new package versions committed (yarn will blow up if the installed packages do not match the ones mentioned in yarn.lock/package.json)

Test Plan:

• Wrote/updated automated tests
• CI tests / new tests are not applicable
• Manually tested my changes, and here are the details:

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 97.13%. Comparing base (d77ae83) to head (5bc36dd).

Additional details and impacted files

@@           Coverage Diff           @@
##             next    #2121   +/-   ##
=======================================
  Coverage   97.13%   97.13%           
=======================================
  Files         114      114           
  Lines        2825     2825           
  Branches      764      764           
=======================================
  Hits         2744     2744           
  Misses         77       77           
  Partials        4        4           

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[github-actions]

size-limit report 📦

Path	Size
components	95.29 KB (0%)
styles	29.85 KB (0%)

[booc0mtaco]

@jeremiah-clothier a quick look at this when you get a sec?