Create a guide for adding new taps/targets to the hub

We can add a guide and link to it at the bottom of the tap, targets, extractors, and loaders paths.

Content to include: https://meltano.slack.com/archives/C01PKLU5D1R/p1629495742038500

I think this should always include the meltano version so that newly added taps/targets are immediately discoverable by meltano.

Which would make three files to create for taps:

• _data/meltano/extractors/<sourcename>.yml
• _data/taps/<sourcename>.yml
• assets/logos/taps/<sourcename>.png (assuming this is the first of this source/destinations)

Or for targets:

• _data/meltano/loaders/<destname>.yml
• _data/targets/<destname>.yml
• assets/logos/targets/<destname>.png (assuming this is the first of this source/destinations)