Rewrite our documentation structure page

[sgibson91]

Context

This page that aims to describe how our documentation is structured feels a little bit outdated https://infrastructure.2i2c.org/en/latest/structure.html

For example, it does not reference our new Hub Deployment Guide or SRE guide, and it references audiences that likely are not visiting these particular docs (perhaps it was written before we had docs.2i2c.org?)

Proposal

Give the page a refresh to reflect the current docs structure so folk can more easily orient themselves in the docs

Updates and actions

No response