Mention (and link to) our Slack, and perhaps also to the GItHub Discussions page

[fingolfin]

Of course we should perhaps just overhaul the whole website content to begin with...

• https://gap-system.org/slack
• https://github.com/gap-system/gap/discussions

[fingolfin]

See gap-system/gap#4690

[fingolfin]

Actually we also should point to the developer mailing list (see https://lists.uni-kl.de/gap/info/gap)