Added intro, single-source tools and "What's in Which Python" sections.

These were the separate commits that conflicted each other in earlier PRs. I've combined them into one PR (on an updated master) so there should be no conflicts.
The way you've written it in your second comment above is quite readable. I will use that, or a close paraphrasing of it. By "modernise", did you mean "python-modernise" here: https://github.com/mitsuhiko/python-modernize