Update infrastructure index page to reflect the new structure

I'm updating the infrastructure page to reflect all the changes that had been happening there, and also introducing an explicit rule of who has access to production and where/how our monitoring is available.

I removed the performance specialist role that is gone, and I reduced the scope of production engineering to the production team.

The rest of the leads should include a description to what their teams do.

cc/ @gl-infra for feedback

cc/ @sytses because of being the director of infrastructure