Scan docs for incorrect references to "customer" and adjust messaging

In several places in our docs we refer to users as "customers" - this does not align with our open source position. Not all users are customers, and we must not make it sound like we're only building content for paying users, because we care about all of our users.

Note: this usage may still be technically correct on occasions, for example in billing-related content. This issue is not to replace every single occurrence, just to limit its usage to required scenarios only.