[DOCS] Add cross domain overview and maurelian's bridge smart contract diagram to the docs

[sbvegan]

Brief Description of the Docs Request

Add this content to the docs: https://warpcast.com/maurelian.eth/0x376c06fd

Description of the Documentation You'd Like

I think it would be best if we carved out a new, "Cross Domain" section under OP STACK > Protocol. We can put a new cross domain overview page with this content there and put the Deposit Flow, Tx Flow, and Withdrawal Flow under that section.

Links to Related Pages

https://warpcast.com/maurelian.eth/0x376c06fd
additional content https://youtu.be/9DMlWFMZ380?feature=shared

Additional Context

He shared a link with us internally

[cpengilly]

@maurelian could you please DM me in slack with the content for this new page? I can't seem to find the original thread. Thanks! 💟

[richardgreg]

Would like to take up this once #853 gets merged 🙂