cmake: add `sptag-` prefix to the names of the binaries

[pabs3]

When building and installing using cmake, the binaries have very generic names like client and server. When installing to paths like /usr/bin, /usr/local/bin, /opt/bin or ~/bin the binaries aren't obviously associated with SPTAG. I suggest adding an sptag- prefix to the names of the binaries, so that it is obvious what they are when installed into a directory that is in the $PATH environment variable.

[pabs3]

The fixes for this issue are in AnnService/CMakeLists.txt on the calls to the add_executable, target_link_libraries and install commands. In addition the documentation in docs/GettingStart.md and docs/Parameters.md will need to be adjusted. I also noticed that docs/GettingStart.md incorrectly uses MixedCase for the names of the binaries instead of lowercase and that docs/GettingStart.md misspells indexbuilder as IndexBuiler.

[pabs3]

Also the GPU binaries need to be adjusted in GPUSupport/CMakeLists.txt.