fix broken links in mongodb connector page (#392)

The documentation link on the MongoDB connector page is broken, so I
updated it.

The prerequisites sections has broken links to steps on creating a
supergraph and a subgraph. The docs are structured differently now so I
didn't see clear pages to switch those links to. Plus the connector
documentation link now covers all of the prerequisite steps so I thought
it would be best to remove the prerequisites section from the hub.

I also switch the issues link to point to the connector repo instead of
the engine repo.

---------

Co-authored-by: Rob Dominguez <[email protected]>

registry/hasura/mongodb/README.md

@@ -8,16 +8,9 @@ which can expose and run GraphQL queries via the Hasura v3 Project.
 The connector implements the [NDC Specification](https://hasura.github.io/ndc-spec/overview.html),
 but does not currently support mutations, column relationship arguments in queries, functions or procedures.
 
-## Prerequisites
-
-1. Create a [Hasura Cloud account](https://console.hasura.io)
-2. Please ensure you have the [DDN CLI](https://hasura.io/docs/3.0/cli/installation) and [Docker](https://docs.docker.com/engine/install/) installed
-3. [Create a supergraph](https://hasura.io/docs/3.0/getting-started/init-supergraph)
-4. [Create a subgraph](https://hasura.io/docs/3.0/getting-started/init-subgraph)
-
 ## Using the MongoDB connector
 
-Check out the [Hasura docs here](https://hasura.io/docs/3.0/getting-started/build/connect-to-data/connect-a-source/?db=MongoDB) to get started with the MongoDB connector.
+Check out the [Hasura docs here](https://hasura.io/docs/3.0/getting-started/with-mongodb) to get started with the MongoDB connector.
 
 ## Troubleshooting