Add Salesforce docs for setting up a connection

Description

As of the related issue on adding a custom connection (see issue link below) we need a how-to guide on setting up a connection to salesforce.

To be more specific we need to add a salesforce.rst to the docs/howto/connection directory explaining the fields you need to set to setup a proper connection to salesforce. A connection will be established through the airflow/providers/salesforce/hooks/salesforce.py hook. Check out this file to see how the connection is established and describe this in the guide you are adding. You can check other how-to connection guides in the directory to see how it works. If you have further questions do not hesitate to ask. 😃

Use case / motivation

If you simply want to connect to salesforce services you should be able to do so without investigating the code and instead just read a proper howto docs explaining what to do.

Related Issues

https://github.com/apache/airflow/issues/8766

[eladkal]

@kaxil I can prepare a PR to explain how to setup connection to Salesforce. It’s relatively short explanation. Once we’ll have more information or a dedicated connection for Salesforce I’ll update the docs accordingly.

[feluelle]

We can perhaps implement alternative auth mechanisms shown here that don’t require additional sensitive data to be passed: https://github.com/simple-salesforce/simple-salesforce/blob/master/README.rst

To login using the JWT method, use your Salesforce username, consumer key from your app, and private key:

from simple_salesforce import Salesforce
sf = Salesforce(username='[e-mail]', consumer_key='XYZ', privatekey_file='filename.key')

But at the moment this is the only working option we have. In my opinion we should add the docs but make sure we have a note that makes it clear that this is/can be a security issue.

What do others think? @potiuk @kaxil ?