home redesign

[DavidLeoni]

Currently, the home in index.ipynb is a blob of text which also contains the manually written toc: it's functional to the purpose, but not very attractive.

It also has the problem that links to topics point to its header like

https://en.softpython.org/#Worked-project

which I suspect it's not great for SEO optimization - see also stable header ids #80

Consider using nbsphinx galleries