Document more clearly how to use Meltano with unknown (but not totally custom) Singer taps and targets

https://meltano.com/tutorials/create-a-custom-extractor.html currently assumes the entire extractor is custom and brand-new, but it's far more likely that users will want to use Meltano with existing taps from https://www.singer.io/, that simply aren't known to Meltano yet.

How to go about this can be documented much more clearly, beyond what's under https://meltano.com/#meltano-add.

As part of this documentation, we can also describe how to go about contributing these changes back to Meltano itself, to make these taps known for all users.