Reorganize filesystem and I/O docs in stdlib

Here's an attempt at making it easier to find filesystem and I/O related functions. This makes the first page (which appears as "The Standard library") shorter, which is good IMHO as it's really too large ATM. I've harmonized headline levels which were a bit messy. Also, I've regrouped filesystem and network functions which appeared related to me, but were in completely different places.

There's something I don't know how to fix: it's that the first page (file base.rst) appears as "The Standard library", when it should be called "Base functions" or "Essential functions". It's very confusing because you don't understand what's the hierarchical organization of the docs and of the stdlib: are other files parts of the stdlib or not? What should I expect to find in that first page (which is the biggest section)?
What's the difference between `module` and `currentmodule`?