docs: link to package host docs from dep search & extend pkg host docs

Created by: mrnugget

This links to the relevant integration pages from the dependency search page, based on this feedback. (cc @emchap @maaaaaaaax)

It also extends the separate integration pages to include explicit "Setup" steps and to also include the schema:

That makes them really similar to the standard code host doc pages we have: https://docs.sourcegraph.com/admin/external_service

Next step should probably be to migrate these pages to admin/external_service and link to them from the integration page. Didn't want to do it all in one PR though.

Test plan

• Tested locally with sg run docsite and looking at pages.