FR: Add HTML partial for Appendix.

Discussed in https://github.com/quarto-dev/quarto-cli/discussions/8632

^{Originally posted by batpigandme February 7, 2024}

Description

A bit related to #6552 in terms of wanting more cutomization, but a bit more minimalistic (i.e. would still be an html partial): Would it be possible to have an HTML template partial for the Appendix (the current <div id="quarto-appendix">)?

I find myself wanting to make slight changes to that section, similar to the kind of tweaks I've made with the title-block.html partial, but, since the appendix is in $body$, I don't have easy access to the variables used.

For example, I'd like to employ the wording from distill's reuse-note, but I think I'd have to make an entirely new template in order to do so right now.

Thanks for your consideration, and/or any workarounds that currently exist but I haven't figured out!