docs: move the README files in the codebase to documentation visible on the web

[MarieSacksick]

Closes skrub-data#991

There are currently 6 readmes in the repo:

number	place	action
1	root	No point to reference it in development
2	skrub/_reporting/readme.rst
3	skrub/_reporting/_data/templates/icons/README.md	could stay where it is. It's quite anecdotal therefore it doesn't seem useful to reference it in development documentation?
4	examples/README.txt	empty --> removed
5	benchmarks/readme.md	can be adapted and the reference can be added to development section
6	benchmarks/results/README.md	Could be deleted in my opinion, because the text is quite short and be squeezed with the other readme about benchmarks. All the more than now that there is a couple of examples, it's straightforward.

Waiting for skrub-data#1857 to be closed before.