tools/scylla-sstable: reorganize main help text
tools/scylla-sstable: reorganize main help text

Currently the main help is a big wall of text. This makes it hard to
quickly jump to the section of interest. This patch reorganizes it into
clear sections, each with a title. Sections are now also ordered
according to the part they reference in the command-line.
This should make it easier for answers to questions regarding a certain
topic to be quickly found, without having to read a lot of text.

scylladb/scylla,scylladb/scylla,scylladb/scylla,scylladb/scylla