[feat][workflow] add instructions on "How to Contribute Connector"

### Search before asking

- [X] I searched in the [issues](https://github.com/apache/pulsar/issues) and found nothing similar.

### Motivation

### Issue

Currently, we do not have instructions on how to contribute a connector.

Devs might forget to add a connector to the [download page](https://pulsar.apache.org/download/) after they finish the development (e.g., https://apache-pulsar.slack.com/archives/C5Z4T36F7/p1676570833081629).

Even though users can check and download all connectors on the [archive page](https://archive.apache.org/dist/pulsar/), we still want to provide complete info to users in quickest time.

### Solution

We need to document the connector contribution workflow. It might contain the following steps:

1. Code: How to submit the code?
2. Doc: How to write a doc for it?
3. Download: How to add it to the download page?

### Alternatives

_No response_

### Anything else?

_No response_

### Are you willing to submit a PR?

- [ ] I'm willing to submit a PR!

Review the existing connector contribution process, the Pulsar download page, and the archive page linked in the issue. Document the code submission, connector documentation, and download-page update steps; done means contributors can follow the workflow and users can find newly released connectors promptly.

Written by the indexing model from the issue text.