Rework Joe docs

The Joe section in the documentation is reworked:

• the wording is polished,
• new subsection added, Reference, and the list of commands migrated there, becoming a separate page,
• an attempt to use video (YouTube embedded) instead of animated GIF.

Checklist

• the MR description has been reviewed
• this MR contains text changes and they have been reviewed OR there are no texts changes
• this MR contains GUI/CLI changes and they have been reviewed OR there are no GUI/CLI changes
• this MR contains API changes and they have been reviewed OR there are no API changes