docs: Simplify images generation to allow parallel build

- remove the tidy target from the main target.
  * aux/log file are already excluded in a .gititgnore file
  * allow parallel generation as the tidy target imposes sequential build

docs/source/_images/Makefile

@@ -1,4 +1,4 @@
-all: examples all_tex tidy
+all: examples all_tex
 
 # set a fake time in pdf generation to prevent unnecessary differences in output
 FAKETIME := TZ='Z' faketime -f '2022-01-01 00:00:00 x0,001'