Fix docutils errors from directives nested in myst-example blocks

[Copilot]

Two docutils build errors occurred from constructs that cannot be nested inside body elements:

• Transition (---) in cross-referencing.md:418 inside a myst-example directive
• contents directive in organising_content.md:31 inside a myst-example directive

The myst-example directive renders content inside divs, but docutils transitions must be direct children of documents/sections, and the contents directive cannot be used within topics or body elements.

Changes:

• cross-referencing.md: Removed --- separator between reference syntax examples (blank line suffices)
• organising_content.md: Replaced myst-example with plain code fence to show contents directive syntax without rendering it

- :::{myst-example}
+ ````md
  ```{contents} Table of Contents
  :depth: 3

• :::

> [!WARNING]
>
> <details>
> <summary>Firewall rules blocked me from connecting to one or more addresses (expand for details)</summary>
>
> #### I tried to connect to the following addresses, but was blocked by firewall rules:
>
> - `api.crossref.org`
>   - Triggering command: `/home/REDACTED/work/MyST-Parser/MyST-Parser/.tox/docs-clean/bin/sphinx-build /home/REDACTED/work/MyST-Parser/MyST-Parser/.tox/docs-clean/bin/sphinx-build -nW --keep-going -b html docs/ docs/_build/html -P` (dns block)
> - `docs.python.org`
>   - Triggering command: `/home/REDACTED/work/MyST-Parser/MyST-Parser/.tox/docs-clean/bin/sphinx-build /home/REDACTED/work/MyST-Parser/MyST-Parser/.tox/docs-clean/bin/sphinx-build -nW --keep-going -b html docs/ docs/_build/html -P` (dns block)
> - `markdown-it-py.readthedocs.io`
>   - Triggering command: `/home/REDACTED/work/MyST-Parser/MyST-Parser/.tox/docs-clean/bin/sphinx-build /home/REDACTED/work/MyST-Parser/MyST-Parser/.tox/docs-clean/bin/sphinx-build -nW --keep-going -b html docs/ docs/_build/html -P` (dns block)
> - `readthedocs.org`
>   - Triggering command: `/home/REDACTED/work/MyST-Parser/MyST-Parser/.tox/docs-clean/bin/sphinx-build /home/REDACTED/work/MyST-Parser/MyST-Parser/.tox/docs-clean/bin/sphinx-build -nW --keep-going -b html docs/ docs/_build/html -P` (dns block)
> - `www.sphinx-doc.org`
>   - Triggering command: `/home/REDACTED/work/MyST-Parser/MyST-Parser/.tox/docs-clean/bin/sphinx-build /home/REDACTED/work/MyST-Parser/MyST-Parser/.tox/docs-clean/bin/sphinx-build -nW --keep-going -b html docs/ docs/_build/html -P` (dns block)
>
> If you need me to access, download, or install something from one of these locations, you can either:
>
> - Configure [Actions setup steps](https://gh.io/copilot/actions-setup-steps) to set up my environment, which run before the firewall is enabled
> - Add the appropriate URLs or hosts to the custom allowlist in this repository's [Copilot coding agent settings](https://github.com/executablebooks/MyST-Parser/settings/copilot/coding_agent) (admins only)
>
> </details>

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<details>

<summary>Original prompt</summary>

> Fix these new issue in the docs build:
> 
> ```
> 2026-01-26T13:16:19.4573013Z �[91m/home/runner/work/MyST-Parser/MyST-Parser/docs/syntax/cross-referencing.md:418: WARNING: Transition must be child of <document> or <section>. [docutils]�[39;49;00m
> 2026-01-26T13:16:19.4575503Z �[31m/home/runner/work/MyST-Parser/MyST-Parser/docs/syntax/organising_content.md:31: ERROR: The "contents" directive may not be used within topics or body elements. [docutils]�[39;49;00m
> 202
> ```


</details>



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