Improve Salesforce Docs

### Tell us about the documentation you'd like us to add or update.
Our Salesforce documentation makes it difficult to understand how to retrieve Client ID, Client Secret and Refresh Token. It's hidden behind an external link to [this walkthrough ](https://medium.com/@bpmmendis94/obtain-access-refresh-tokens-from-salesforce-rest-api-a324fe4ccd9b) which means when a user tries to CMD + F for keywords like `client id` they aren't able to find it. We should include these keywords directly in the doc or move the walkthrough into the doc completely.

### If applicable, add links to the relevant docs that should be updated
https://docs.airbyte.com/integrations/sources/salesforce/