Keep an eye on `myst-parser` and `myst-nb` updates

**Is your feature request related to a problem? Please describe.**

The `myst-parser` package went to version `1.0.0` a couple of months back and so some of syntax and features should now be stable. Notably, we don't use `colon-fence`s much, and those now seem like canonical syntax.

At the moment, we're currently pinned to `myst-parser <1.0.0` by some `myst-nb` and `sphinx` version pins for using MyST documents as notebooks and as Sphinx content ( see https://github.com/executablebooks/MyST-NB/pull/513). As and when that is resolved we should review our MyST content and syntax.

**Describe the solution you'd like**

Review the MyST docs to update syntax to 1.0.0

Start by checking the repository's MyST documentation and the myst-parser/myst-nb version constraints mentioned in the issue. Review the MyST content for syntax changes once those pins are resolved; done means the relevant documentation uses syntax compatible with myst-parser 1.0.0.

Written by the indexing model from the issue text.