Handle documentation installation separate from binary installation.

[okeuday]

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This change is basic/small, though you may want to handle the makefile dependencies in a different way (this is just the simple approach that doesn't break anything). The reason for this change, is to make sure tsung can be packaged easily, so there is clear delineation between documentation and binary installation files and makefile targets.

[tisba]

This looks simple and straight forward enough. What do you think @nniclausse?

[tisba]

The change looks okay to me, but I'm not sure how your release process works and if this change interferes with that. Can you have a quick look, @nniclausse?