docs: replace `runserver` with `start`

[Carlos-Muniz]

Part of https://github.com/overhangio/2u-tutor-adoption/issues/26

In order to remove runserver, the following must be done for all official tutor plugins:

• [X] Ensure that the plugin is ported to the V1 API
• [X] Ensure that the plugin hooks into the COMPOSE_MOUNTS filter in order to automatically bind-mount folders as appropriate.
• [x] Replace any references to runserver with start
• [X] Replace any references to the --volume option with an equivalent usage of the --mount option.

Within this repo, there was only one incomplete task: 'Replace any references to runserver with start'. This task is solved with this PR.

[Carlos-Muniz]

@kdmccormick @regisb This is ready for review