Migrate YAML docs

[speth]

Changes proposed in this pull request

• Migrate YAML-related documentation into the versioned Sphinx documentation
• Re-organize this content a bit, within the "User Guide" section of the website
  • A short introduction, syntax overview, and discussion of included input files is given in the "Introduction to YAML Input Files"
  • Under the "task guides", we have:
    • "Converting Chemkin Format Files"
    • "Creating YAML Mechanism Files from Scratch" (which has been condensed to a single page),
    • "Converting Data from NASA ThermoBuild"
    • "Handling Errors in Input Files"
    • "Converting Legacy CTI and XML Input Files to YAML"

If applicable, fill in the issue number this pull request is fixing

Continues work on Cantera/enhancements#178.

Checklist

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

[codecov]

Codecov Report

Merging #1638 (db3fc12) into main (3a9bc6a) will decrease coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main    #1638      +/-   ##
==========================================
- Coverage   72.69%   72.68%   -0.01%     
==========================================
  Files         370      370              
  Lines       56298    56298              
  Branches    20375    20403      +28     
==========================================
- Hits        40925    40922       -3     
+ Misses      12376    12369       -7     
- Partials     2997     3007      +10     

Files	Coverage Δ
interfaces/cython/cantera/ck2yaml.py	84.65% <100.00%> (ø)

... and 4 files with indirect coverage changes

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[ischoegl]

Thanks, @speth!