Adding screen recordings to walt docs

[eduble]

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...

[eduble]

To make it most simple, we could also just reuse walt help show <topic>, and the command would do what is needed if the <topic> is a screen recording or not.

We will have to take care with the markdown to html conversion code (doc/walt/doc/sphinx/conf.py) which looks for cross-references in the form of walt help show <topic>, and currently replaces all of them with internal web links. In some cases, it could be better to have the player displayed inline on the html page, instead of linking to another dedicated page.