Documentation updates for developers/contributors

[speth]

Changes proposed in this pull request

• Rewrite the Python and YAML API documentation landing pages to provide a prose introduction with links to specific topics of interest. Previously, these were just auto-generated tables of contents
• Add several pages to the Develop - Adding New Features to Cantera section of the documentation
  • Integrate the contents of CONTRIBUTING.md into the website
  • Move the code style guidelines into this section
  • New page about writing tests
  • New page about writing examples to work well with Sphinx Gallery
  • New page about running tests and debugging, especially through VS Code
  • New page with general tips for working with VS Code (rescued from the mostly defunct dev-tips repo)
• Update contributing suggestions, PR/git workflow guidance
• Describe method for documenting Cython class attributes
• Use stable version of Sphinx Gallery now that the changes we need have been integrated
• Update dependencies for building docs
• Add reference for PLOG formulation
• Fix some CI issues

Checklist

• [x] The pull request includes a clear description of this code change
• [x] Commit messages have short titles and reference relevant issues
• [x] Build passes (scons build & scons test) and unit tests address code coverage
• [x] Style & formatting of contributed code follows contributing guidelines
• [x] The pull request is ready for review

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 75.69%. Comparing base (d37a76b) to head (20390e7).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #1700 +/- ## ========================================== - Coverage 75.69% 75.69% -0.01% ========================================== Files 443 443 Lines 60975 60970 -5 Branches 9551 9551 ========================================== - Hits 46158 46153 -5 Misses 11786 11786 Partials 3031 3031 ```

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