notebook intro at help-video.html does not exist

[haraldschilly]

There is a link on https://www.sagemath.org/help-video.html to https://www.mosaic-web.org/MCAST/videos/MCAST-2011-03-18/lib/playback.html … going into the void

Since the interface is jupyter, we probably just remove this section.

[mkoeppe]

See also https://groups.google.com/g/sage-devel/c/vh2Yu7c93Zs/m/o_r9msxsAgAJ – I have collected some links to working videos at https://trac.sagemath.org/wiki/SagePresentationsMarketplace, some of which could be used as replacements