Clean pkgdown website on releases

[Bisaloo]

This avoid keeping obsolete vignettes that have been deleted from the main branch. I believe it's safe and a good trade-off to clean on releases.

Related: #484

I have added the following comment in my internal workflow and can copy it here if you believe that'd be helpful:

# We clean on releases because we want to remove old vignettes, 
# figures, etc. that have been deleted from the `main` branch.
# But we clean ONLY on releases because we want to be able to keep
# both the 'stable' and 'dev' websites.
# Also discussed in https://github.com/r-lib/actions/issues/484

[codecov-commenter]

Codecov Report

Merging #735 (494d68a) into v2-branch (5ab5909) will not change coverage.
The diff coverage is n/a.

❗ Current head 494d68a differs from pull request most recent head 722bdba. Consider uploading reports for the commit 722bdba to get more accurate results

@@             Coverage Diff             @@
##           v2-branch      #735   +/-   ##
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  Coverage     100.00%   100.00%           
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  Files              2         2           
  Lines             10        10           
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  Hits              10        10           

[gaborcsardi]

That's a good idea, but if you have also have a pkgdown site for the dev version of the package, that is also cleaned up, unfortunately.

[Bisaloo]

Ah yes, I assumed the case where the release is created immediately and it is perfectly in sync with main. In this case, it makes sense to not have a dev version.

One potential solution would be to trigger again the workflow on HEAD if it was triggered by a release. But it might start to be a little bit too involved 🤔

[github-actions]

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