Use heading 2 for subsections of Timing dev docs

[andrewdnolan]

Switches to heading 2 for subsections and heading 3 for sub-subsection in the Timing dev docs.

Fixes #397.

Checklist

• Documentation:
  • Documentation has been built locally and changes look as expected
• Linting
  • Pre-commit run locally

[andrewdnolan]

Testing:

Documentation can be found here: https://portal.nersc.gov/cfs/e3sm/anolan/omega_PR398/_build/html/

Looking at the table of contents on the LHS, it seems like the switch to heading 2 works as expected:

[xylar]

Thanks @andrewdnolan!