Try to speed up local docs rebuild by avoiding re-copying and processing assumed-unchanged classic docs data
See: https://lists.mcs.anl.gov/pipermail/petsc-dev/2022-January/027973.html
If already assuming that a local "classic" docs build is current, it may be possible and simple to just check if the destination for the man pages and HTML source exists from a previous build, and thus avoid copying the files (which are over 100MB, so not trivial) and adding the version number to the man pages.