[examples][tracer] Choose filenames using a naming scheme

Now that we are doing this so professionally with the examples, I have one more small thing to pick on. This introduces a naming scheme for the markdown files.

• Generated subpages have some simple name preferably without underscores e.g. 1p.md (1pmodel.hh was a bit confusing I thought because the doc actually contains a problem and spatial params)
• Parts of subpages are named subpage_subtitle, e.g. 1p_intro.md
• Parts of the main page are named _subtitle, e.g _intro.md

ping @utz