doc builder: Improve verbosity filter

[samsrabin]

Description of changes

Resolves issues @slevis-lmwg experienced here:

• CRITICAL messages from Sphinx build were getting filtered out in non-verbose mode
• Spurious warning message about image platform not matching expected was not getting filtered out.

This led to it seeming like the image platform thing was the problem.

The new version of doc-builder, when not verbose:

• Includes CRITICAL messages
• Suppresses "image platform does not match expected" warning

So before:

Cleaning documentation build directory...
Done.
Building documentation...
WARNING: The requested image's platform (linux/amd64/v8) does not match the detected host platform (linux/amd64/v3) and no specific platform was requested
Documentation build completed, but with problems that must be resolved.
Re-run with --verbose for full output.

After:

Cleaning documentation build directory...
Done.
Building documentation...
/home/user/mounted_home/doc/source/tech_note/FUN/CLM50_Tech_Note_FUN.rst:4: CRITICAL: Duplicate ID: "equation-n-cost-fix". [docutils]
Documentation build completed, but with problems that must be resolved.
Re-run with --verbose for full output.

Specific notes

Contributors other than yourself, if any: None

CTSM Issues Fixed (include github issue #): None

Are answers expected to change (and if so in what way)? No

Any User Interface Changes (namelist or namelist defaults changes)? No

Does this create a need to change or add documentation? Did you do so? No

Testing performed, if any: Manual testing.

[slevis-lmwg]

Thanks @samsrabin.