Editorial: find a new home for "fetch params" and friends

[annevk]

We put "fetch params" in the Infrastructure section temporarily, but I think it all needs to move to the Fetching section.

I think it should first have a "Fetching miscellaneous" section that contains "fetch params", "fetch controller", etc. And then a section "Fetch algorithm" that is essentially what "Fetching" starts with now.

And then ideally we'd have a "Fetching introduction" section at some point (either named or implicit) at the start that gives a brief overview and perhaps has a diagram with the control flow.

(See #1329 for context.)