[Backup] Correctly install/load documentation for the tools separate package

What's the issue?
The tools separate package currently doesn't contain any of our documentation (at least I don't believe it does, we should double check)?

All the tools in 'backup' will load documentation when the user uses the 'help' command or '--help' flag.

At the moment, it's likely that this would fail, as we'd fail to find/open the documentation.

What's the fix?
We should install the documentation into the standalone package (probably) using the docs/CmakeLists.txt file.

We should update logic so that we can correctly open the documentation in this new location; this will likely require extending the compile time options to set the type of install.

https://github.com/couchbase/backup/blob/30685d694149f500455430863ba2f219743c5b93/docs/CMakeLists.txt
https://github.com/couchbase/backup/blob/30685d694149f500455430863ba2f219743c5b93/man/man.go#L41-L45
https://github.com/couchbase/backup/blob/30685d694149f500455430863ba2f219743c5b93/man/man_unix.go#L15-L21