Add a tutorial section to the page

[tmadlener]

BEGINRELEASENOTES

• Add a tutorial section to the page.
• Move existing starterkit documentation into the tutorials section

ENDRELEASENOTES

@jmcarcell @andresailer @gaede as discussed I added a tutorial section to the page. Currently there is an empty "example" tutorial just to demonstrate the layout in the end.
This looks like this in the current page:

I will start to add the converter tutorial to this in a separate PR. I think we could potentially merge this first (or at least everyone can start from here). For adding a new tutorial you simply need to add the markdown or rst file into the eval_rst block in the README.md.

[tmadlener]

I have realized that we already have quite a bit of tutorial style content on the page, so I have effectively just shuffled that into the Key4hep Tutorials category. I had to adapt the fetch_external_sources.sh script slightly to not only work on the toplevel README.md. This still allows us to put content into this repository but also fetches external content from others, so we don't have to duplicate things.

It now looks like this:

[tmadlener]

@jmcarcell @andresailer anything against merging this so that we can continue on top of this for the Paestum tutorial?

[jmcarcell]

I think this is fine