doc: Add a page for conduit connector developers outlining best practices for writing unit tests

@nickchomey suggested that our documentation could benefit from a dedicated page on best practices for writing unit tests on Conduit Connectors.

Including an example would be helpful for external contributors, so we should strive to make it as easy as possible for them.

Start by locating the existing connector documentation structure and its unit-testing guidance in this site, then compare nearby pages for format and examples. Add a dedicated page covering best practices for writing unit tests on Conduit Connectors, including an example that helps external contributors, and verify it builds with the site's documentation.

Written by the indexing model from the issue text.