Document the new `format: typst` template variables

[cwickham]

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Description

PR #13249 introduced several new variables into the default Typst template.
These need:

1. Schema entries in quarto-cli so they appear in the auto-generated Reference pages
2. Sections in Typst Basics docs/output-formats/typst.qmd

The new variables:

• mathfont, codefont, linestretch
• linkcolor, citecolor, filecolor
• thanks, abstract-title