Doc: extension-to-extension provider connections

[kumavis]

Following up on #3997

here is implementation
https://github.com/jakubsta/example-extension/blob/master/background.js#L7

[kumavis]

beyond documentation we may want to publish this provider

[bdresser]

mostly handled https://medium.com/@writeprovidence/how-to-create-a-web3-provider-from-another-extension-6b5ea59ed4c9 and https://github.com/MetaMask/metamask-extension-provider