Encourage using an env variable for stateSecret in documents

[seratch]

There is no reason not to use an env variable for the stateSecret:

The sample code in the docs for Bolt Oauth setup has the stateSecret exmple in single quotes as text, whereas for the client id or client secret it puts it as an env variable. Is there a reason that the stateSecret isn't an env variable? Is the stateSecret supposed to be generated randomly each time the app install process is called and so should be generated in the code itself every time? Thanks!

Originally posted by @anthonygualandri in https://github.com/slackapi/bolt-js/issues/701#issuecomment-1009472833