Improve description of how to define and use custom elements

[speth]

The description of how to use custom elements within a phase definition is a bit confusing (see a recent Users' Group post). I think this is mainly due to two things:

1. the lack of a complete example that includes both the top-level elements section where the new elements are defined as well as the elements field within the phase definition.
2. The fact that on the page describing phase definitions (https://cantera.org/tutorials/yaml/phases.html), the examples show partial phase definitions without the outer structure, making them look like top-level sections, e.g.

   elements:
   - default: [C, H, Ar]
   - isotopes: [O18]
   - myelements.yaml/uranium: [U235, U238]