Docs: Update references to Slack channel names

[lauriemerrell]

As a Cal-ITP docs user, I want any reference to Slack channel names to be current so that it's clear what channel is being referenced.

With the current Slack organization effort, several channel names have been changed. We should audit docs.calitp.org in particular and the repo in general for any references to Slack channel names and update names that are out of date.