Make ck2yaml docs easier to find

[speth]

In addition to linking the docs for ck2yaml (https://cantera.org/tutorials/ck2yaml-tutorial.html) from the "Tutorials" section on input files (https://cantera.org/tutorials/input-files.html), they should also be linked from the "Defining Phases" guide to Cantera's YAML format (https://cantera.org/tutorials/yaml/defining-phases.html)

[speth]

This is already done as part of the recent website reorganization.