[mystmd] Fix mystmd build warnings

[kp992]

⚠️  about_py.md:419 unexpected option "scale" provided (in figure)
⚠️  about_py.md:425 unexpected option "scale" provided (in figure)
⚠️  getting_started.md:370 unexpected option "scale" provided (in figure)
⚠️  python_by_example.md:39 unexpected option "scale" provided (in figure)
⛔️ python_oop.md:12 Too many colons encountered in index line "single: OOP II: Building Classes"
   Index entry must follow pattern "type: entry; sub entry"
⛔️ python_oop.md:12 No entries parsed from index directive
⚠️  matplotlib.md:454 unexpected option "scale" provided (in figure)
⚠️  pandas.md:38 unexpected option "scale" provided (in figure)
⚠️  pandas.md:677 unexpected option "scale" provided (in figure)
⚠️  pandas.md:743 unexpected option "scale" provided (in figure)
⚠️  parallelization.md:136 unexpected option "scale" provided (in figure)
⛔️ status.md:16 unknown directive: nb-exec-table

@mmcky I have fixed almost all the warning apart from the few above. I am not sure how to migrate scale argument of figure in mystmd. Also, any suggestions on how to fix the status.md?

[kp992]

Edit, the warnings are now reduced to:

⛔️ python_oop.md:12 Too many colons encountered in index line "single: OOP II: Building Classes"
   Index entry must follow pattern "type: entry; sub entry"
⛔️ python_oop.md:12 No entries parsed from index directive
⛔️ status.md:16 unknown directive: nb-exec-table

[mmcky]

Consolidating into #593, which now carries this issue's two remaining warnings (python_oop.md index entry, status.md nb-exec-table) as checklist items — one tracker for the whole migration. Closing in its favour.