Introduce a "Map of NiceGUI"

[falkoschindler]

This PR adds an overview of the NiceGUI namespace to the documentation page.

• It's still a draft, because I'm planning to add links to the individual documentation pages where possible.
• And although I like the blue color to highlight the code snippets, it can be confused with links. So maybe we find a better way to highlight them.

[rodja]

You modified the default css styling. Probably to enhance readability. But now the two kinds of links are intermixed:

I think we should not alter the link styling: