make view page source refer to gitlab repo, improve versioning

currently it includes the src in the actual doc, make it so it points to:

https://git.mel.vin/vermim/libxmpp/blob/master/doc/index.rst

this should however not be hardcoded to master branch, but taken from current build

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while we are at it, all non-tag doc builds should have a suffix version, probably the short commit id or the slug. this should work locally too so CMake needs to invoke git and check if the current commit is a tag

also check is the local tree is clean, if not append a + to the version suffix just like Linux kernel does