Restore capability to export metadata sections (previously headers) to html

[climbfuji]

Restore capability to export metadata sections (previously headers) to html.

The capability to write metadata sections to html was removed with the refactoring of the feature/capgen branch, subsequently slipped the reviews and got merged into main. As a result, the metadata2html.py script, required to generate scientific documenation (especially at times of releases!) doesn't work anymore. This PR restores the capability by putting the original code blocks back into the metadata sections (previously these were called headers).

I ran this in the two supported ways within the UFS. Once merged, the code will work like this. Maybe there are some slight path changes compared to what is in the CCPP tech doc - @mkavulich FYI.

Batch mode using ccpp_prebuild_config.py:

git clone -b develop --recursive https://github.com/ufs-community/ufs-weather-model
cd ufs-weather-model/FV3/ccpp
PYTHONPATH="$PWD/framework/scripts:$PWD/framework/scripts/parse_tools:$PYTHONPATH"  \
    ./framework/scripts/metadata2html.py -c config/ccpp_prebuild_config.py

Single-file mode:

mkdir test
PYTHONPATH="$PWD/framework/scripts:$PWD/framework/scripts/parse_tools:$PYTHONPATH" ./framework/scripts/metadata2html.py -m physics/physics/mp_thompson.meta -o test

User interface changes?: Yes (I assume some of the paths have changed compared to the documentation)

Closes #444

Testing: see above. The code changes will also be tested within the ufs-weather-model regression testing environment. I recommend that we pull this PR into #443, since #443 and associated PRs do not change the baselines. Note that #444 is required for CCPP release v6.

Other testing:

• for ccpp_prebuild.py:

  # relative to above directory
  cd framework/tests
  PYTHONPATH="$PWD/../scripts:$PWD/../scripts/parse_tools:$PYTHONPATH" python3 test_metadata_parser.py
  PYTHONPATH="$PWD/../scripts:$PWD/../scripts/parse_tools:$PYTHONPATH" python3 test_mkstatic.py 

• for capgen.py:

  # relative to previous `tests` directory
  cd ../test
  ./run_tests.sh
  # All tests PASSed!
  ./run_doctests.sh
  # No output, which means no error - all tests PASSed

  test removed: none unit tests: none system tests: none manual testing: see above

[climbfuji]

@mkavulich @grantfirl I addressed @gold2718's review comments, now it is your turn ...

[grantfirl]

@climbfuji Could we combine this with #443 for the purposes of merging with UFS?

[climbfuji]

Yes, I was planning to combine #443 #445 and #447 (if approved by then).