Remove unnecessary use of the word "generic"

As requested by DIJ.
Correct wording around HPX in conclusion

This was missed when changing it from HEALPix to HPX.
Correct spelling of Galactic Center

Missed by aspell because we allow "Centre" as part of JAC and CADC.
Edit catalog section

Some parts reordered or rephrased a bit.

Added an introductory sentence to the island catalog subsection
to explain why we are interested in the noise.