[DOCS] Add information about `proxyd`

[sbvegan]

Brief Description of the Docs Request

Add information about the proxyd service: https://github.com/ethereum-optimism/optimism/tree/develop/proxyd

Description of the Documentation You'd Like

I think our OP Stack section would benefit from a sub-section dedicated to operators.

Links to Related Pages

Doesn't exist, but I think this should go under OP Stack, in its own subsection.

Additional Context

We're going to want to add more OP Stack operator run books and documentation. I think this would be a good opportunity to start carving out that space.