[revealjs] Mention `title-fancy/title-slide.html` as the partial starter to use for the title slide.

[cderv]

This partials is the default one for Quarto using authors and affiliations normalization. This is the one to use as a starter.

Introduced initially in https://github.com/quarto-dev/quarto-cli/commit/9e280991de9cefac47d2088bae4ee620fa752e2a

Closes https://github.com/quarto-dev/quarto-cli/issues/10308

@cwickham asking for review for wording and other style and form comment. We can also improved all this when dealing with

• https://github.com/quarto-dev/quarto-cli/issues/5194