main/umurmur: add umurmurd-doc subpackage

These are manual pages from the OpenBSD port, since upstream doesn't ship any. They don't include any OpenBSD-specific details anyway.

assigned to @larena

added 200 commits

• d8319db1...e6230d15 - 199 commits from branch alpine:master
• 21abc62e - main/umurmur: add umurmurd-doc subpackage

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merged