Add docs/sample for using browser cookies to pass custom app data during OAuth flow

[filmaj]

We have many inquiries from users on how to pass custom data through the Slack OAuth flow (for example, to associate a Slack user with the app's user):

• How to access Company ID during OAuth for Slack App Installation #2123
• Custom attributes/properties to be passed for Slack Oauth #1453

Typically, us maintainers answer these questions by linking to previous other issues/questions where this is explained to users.

Instead, we should write a document on how to do it, and add a sample application showing the technique. In general, the OAuth documentation could use a review and rewrite. Perhaps it is worth pulling in our Education team to help with this effort!