docs: Elaborate on creating custom slides

[matthew-carroll]

Currently, there is a brief article about creating custom slides: https://flutterdeck.dev/slide-templates/custom-slide/

While this guide technically describes how to create a custom slide, I think it's missing a lot of relevant info.

A custom slide eliminates everything that's otherwise a standard piece of another type of slide, e.g., header, footer, theme, layout.

The average reason to create a custom slide is probably that a user wants something similar to an existing slide, but sufficiently different. That means, they probably still want to support the same footer they have elsewhere in the app. They probably want similar theming to the rest of their slides. They just want to mess with the layout a bit.

For that reason, I recommend adding sections to the custom slide guide, which shows users how to re-apply the theme, re-add the header and footer, etc. Show the user how to reconstitute all the aspects of the other slides, in case the user still wants all those details.

Moreover, by doing that in a guide, I think you'll discover that various things need to be exported, which aren't currently exported, as described here: #109