Add documentation about the demo example and clean up examples

[stichbury]

This is a good issue for someone interested in documentation. It doesn't require any particular knowledge of Vizro but does need an interest in poking through our examples folder in within vizro-core for this repo to see what is there and write about the layout and what the example shows. There's a basic style guide for docs creators here, but we are happy to take any contribution as we can run Vale over it to highlight any tweaks needed to follow the guidlines.

The examples folder in the repo holds the code for the demo.

It isn't very well highlighted in the docs and we need to do this:

1. Add a section on the home page and a link to a page of text about the demo (mentioned in Restyle the documentation landing page #378 which will restyle that whole page)
2. Add the page itself. That page should document the basics of the demo and how to work with it.
3. Add a section in the examples folder where any contributions of example code for Create an example dashboard for novice/intermediate/advanced users #377 will be added.