[examples] Better structure and overview by subfolders

Here are some ideas for a better structure in the examples:

Especially if they are large (see !1784 (merged) for an extreme case).

We could

• add separate .md files in the doc folder that are linked from the main document rather than included into the main document
• we could have a separate starting page that explains the setup and the layout, folder and file structure in the example and provides links to the documentation of each file or a group of files
• automate the script to look for .doc_config in all subfolder recursively so that for every subfolder we could have an extra autogenerated README.md if wanted. That README.md can be linked from the main document.
• Remove header guards from the doc
• Improve figure formatting by using HTML

  <figure>
     <img src="dumux_logo" alt="Dumux logo" style="width:50%">
     <figcaption><b>Figure 1 -</b> This is the figure caption for the Dumux logo</figcaption>
  </figure>

  

  Figure 1 - This is the figure caption for the Dumux logo

• see #811 (closed)