Elevate discussion of templates out of Journal Articles

Creating issue to track https://github.com/quarto-dev/quarto-cli/discussions/5134#discussioncomment-5613083

In regards to "Template Partials":

In the Quarto documentation I probably didn't see it because it's under "article templates"?

We have some good docs on using Pandoc templates but they are buried under "Extensions/Journal Articles". This content feels useful beyond that context and should probably exist at a higher level with appropriate linking.

A logical place might be Guide -> Advanced, or covered in the relevant format sections of the guide, like in Revealjs Custom Title Slide.

Article Templates is currently linked to from:

• https://quarto.org/docs/journals/formats.html#example-acm-format
• https://quarto.org/docs/journals/formats.html#format-components
• https://quarto.org/docs/authoring/title-blocks.html#custom-title-pages
• https://quarto.org/docs/faq/rmarkdown.html#can-you-create-custom-formats-for-quarto-like-you-can-for-r-markdown
• https://quarto.org/docs/prerelease/1.4/typst.html#template-partials

Proposed solution:

• Move body of existing Article Templates page to Guide > Advanced. Re-write Overview to be more general beyond Journal Articles, and Journal Templates become an example of use in body copy.
• Make sure HTML, PDF and revealjs sections of the Guide provides links and possibly specific examples
• Update links and check they still make sense.