The default user in the mysql container is 'mysql',
so the `mysql` command tries to use the 'mysql' MySQL user by default.
But, in the MySQL dump instructions,
we are providing the MySQL root user's password, so we need
to specify `MYSQL_ROOT_USERNAME` as the MySQL user when invoking `mysql`.
There is too much information in each of the local/k8s/dev docs pages. The
"guides" that are listed in each one of those pages are moved either to "common
tasks" or to a dedicated "tutorials" section. This paves the way for more
comprehensive tutorials, where we describe how to run the latest master
branches of Open edX.
I am well aware that, as they stand, the tutorials are of poor quality and
should be rewritten. This is a task for another day/commit. For now, we only
move the contents to a separate part of the docs.
Also, we should add a "reference" section to the docs, where we add the result
of `tutor <subcommand> --help`.
