DOC - Refactor /quickstart guide

ON BEHALF OF https://github.com/polymetric:

It looks like whoever wrote this quickstart guide wrote it without actually testing. Nothing wrong with that, though! I did something like that just the other day. But it should probably be fixed. Cassandra sounds awesome to me but this didn't seem like the best way to give a first impression, so I tried my hand at improving it.

list of changes:

• removed the authentication part as the test works fine without that and is probably a bit out of the scope of this document
• replaced # comments in the CQL script with the proper --
• is now $(pwd) as the former did not work on my system (Pop_OS 20.10, zsh) one of the invocations also had a missing right angle bracket
• the functions toTimeStamp(toDate(now)) threw an error. Apparently, the function toDate() is deprecated? I replaced it with the recommended toTimeStamp(now()) (now was also missing its ()) and ran into a second error - 1234 is not a valid value for type "text." so I added single quotes and voila.
• used a network to connect the database and shell containers because host.docker.internal does not work on Linux
• added another step with a docker command to use cqlsh interactively (there wasn't one before, all the previous command in step 4 did was load data.cql, it did not start an interactive session, which was very confusing)
• rewrote and reorganized some of the text due to the new step

This should be tested on something other than Linux, because I haven't had a chance to do that.
of course all these changes are subject to evaluation! I probably should've split it out into multiple commits. Hopefully the diff isn't too hard to read. Let me know if I forgot to mention something or you need anything clarified.