DOC: Build documentation manually instead of using sphinx
I wasted several days of my life in the last months trying to understand sphinx code, and seeing how make it perform simple things like changing the depth of the sidebar table of contents, add breadcrumbs, and similar things.

Not sure what went wrong with sphinx development, but its code lacks any reasonable architecture, and the complexity for what we make it do is huge compared to what it should be.

So, given that the only two options I see are give up and have a suboptimal website for pandas, or stop using sphinx, I decided to start working on the latter.

There will surely be some challenges, but the core of what we need is simply:
- Extract information from `.rst` files (`docutil` and `numpydoc` will do all or most of the job in this)
- Render the extracted content to html templates (`jinja2` will do that)
- Manage the cross-references among documents (that will need to be implemented, but doesn't seem so much work or so challenging)

I'll work in this as a separate reusable project, that  can be imported as a dependency.

CC: @jorisvandenbossche @TomAugspurger