add metamask proxy and cross window provider docs

[andreigiura]

Description of the pull request (what is new / what has changed)

Did you test the changes locally ?

• [x] yes
• [ ] no

Which category (categories) does this pull request belong to?

• [x] document new feature
• [ ] update documentation that is not relevant anymore
• [ ] add examples or more information about a component
• [ ] fix grammar issues
• [ ] other