Update several aspects of the alternator/getting-started.md which were
not up-to-date:
* When the documented was written, Alternator was moving quickly so we
recommended running a nightly version. This is no longer the case, so
we should recommend running the latest stable build.
* The link to the download link is no longer helpful for getting Docker
instructions (it shows some generic download options). Instead point to
our dockerhub page.
* Replace mentions of "Scylla" by the new official name, "ScyllaDB".
* Miscelleneous copy-edits.
Fixes#11218
Signed-off-by: Nadav Har'El <nyh@scylladb.com>
Closes#11605
517c1529aa