Improvement to the experience of joining Slack, and getting in contact to get an invite if needed

[dellagustin-sap]

Is your feature request related to a problem? Please describe.
When new members try to join our Slack workspace, they are only allowed if their email belongs to a list of allowed domains.

If the domain of their email is not allowed, they need to contact the workspace administrator at InnerSourceCommons for an invitation.
The contact information can be found under https://innersourcecommons.org/about/contact/, which is kind of hidden under "About" or at the page footer.

Describe the solution you'd like
To make it easier for new members to get in contact if needed, I suggest to add the text "Contact us if you have trouble joining" below the button to join Slack at https://innersourcecommons.org/community/.

Additional context