Docu speedup

This fixes the following faults:

• set_inheritance_diagramm.py inserts the diagramm multiple times in the same file if run mutliple times
• set_inheritance_diagramm.py touches every file when run. Then the next step in the generation has to re-generate all files which takes a few minutes
• The .rst files are not cached

CI Run	Duration of Documnentation Stage	Speedup
Before this MR	2 minutes 37 seconds	-
First run in this MR	2 minutes 33 seconds	-
Successive run with this MR	21 seconds	> 7x