Allow slack registrations links to directly land into specific projects

[kmova]

As projects are moving from individual slacks to CNCF slack, there can be some improvement to registration workflow, for people joining CNCF slack to interact with specific project communities. Most often people join to get their questions answered at the earliest and it will help to remove any barriers in reaching the right community.

One idea could be to present with a welcome message after registering that shows the list of project-specific channels that are available or alternatively, if users are coming to CNCF slack via a Project website, some way to capture that referring site and redirect them to the intended channel after they register.

[amye]

@kmova: Slack doesn't currently allow for just 'individual channel invites' unless you're doing single channel guests, which makes for a bad community experience. Instead, link on your project page to your project channels specifically?