WIP: pkgdown preview on label-only

[pat-s]

forked from #161

[pat-s]

@maelle I've forked #161 to not mingle with your branch state.

Here I essentially just merged main and then tested the label approach.
I think it works great and I am thinking of also integrating it in other projects of mine.

[maelle]

@krlmlr does this approach (pkgdown preview on label only) sound useful for fledge and other packages?

[krlmlr]

Good idea, I'm quite satisfied with preview on pushing to a branch prefixed with docs though.

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