Highlight the "reserved" labels (HTML ids) prefixes

[mcanouil]

Recently and few times already, some issues were reported when using some of the prefixes defined in Quarto for cross-references.

• https://twitter.com/PipingHotData/status/1585067723268050944?s=20&t=QMv2ClFJdoD8W6cWpy0-5A
• https://twitter.com/LisaDeBruine/status/1584586868058132481?s=20&t=QMv2ClFJdoD8W6cWpy0-5A

I think, it would be a good idea to highlight somewhere (maybe in https://quarto.org/docs/authoring/cross-references.html) that there are some "reserved" prefixes for labels and HTML ids, such as fig-, tbl-, thm-, eq-, etc.

[cscheid]

For sure. We'll be doing some major work on those starting on 1.3. One thing we could do is emit a warning if the content of figures/etc doesn't match our expectations. Right now we're silently failing most of the time.

[mcanouil]

Closing in favour of #6201.