Adding screen recordings to walt docs

Some time ago we recorded a first screen recording for showing how to deploy WALT on top of Grid'5000: https://asciinema.org/a/414589.
This recording is also referenced from the walt-project website ("WalT videos" section).

This format can be very efficient in some cases, such as tutorials.

So we could:

• Save this terminal recordings and new ones at doc/walt/doc/cast/.
• Integrate the tool asciinema as a dependency of walt-doc package, and use it to implement a new command walt help play <recording>. asciinema is a pure-python tool, easily installable (no dependency).
• The asciinema project also provides a javascript player, so we could quite easily integrate screen recordings in our html doc (at http://<walt-server>/doc and at https://walt.readthedocs.io). Click the "Sharing" button below the player at https://asciinema.org/a/414589 for more info.

I also wonder if it would be interesting to let the user record its own terminal sessions.
In this case instead of adding a play subcommand to the help category, we could imagine adding a new category called session and adding subcommands such as play, record, list there. The user would still get a set of default terminal recordings (tutorials referenced in the help pages) right after installation, but she could also do its own recordings. If we do that, we could also imagine a publish subcommand then...

Start by reviewing doc/walt/doc/cast/ and the walt-doc package, then inspect the existing help command entry points. Compare the proposed asciinema dependency, walt help play <recording>, HTML documentation integration, and possible session commands. Done is not defined because the issue presents several alternative scopes without selecting one.