doc: move content from conduit repository

[raulb]

We recently moved the content from conduit/docs/writing-a-connector-guidelines¹ to conduit.io/docs/developing/connectors/² for better visibility (we also updated some things that were out of date).

We should consider doing the same for the other relevant documents on https://github.com/ConduitIO/conduit/tree/main/docs.

• Connector discovery
• Connectors
• Health check
• Metrics
• Add link in storage It is empty.

Part of ConduitIO/conduit#1919

Footnotes

1. https://github.com/ConduitIO/conduit/pull/1972 ↩

2. https://github.com/ConduitIO/conduit-site/pull/191 ↩