Add information about the various possible formats in ?list_valid_format_labels() as a new vignette section of rtables:format_precedence.

[martincadek]

          We need to add information about the various possible formats in `?list_valid_format_labels()` and in {rtables} with a looot of examples. If you do `?rtables::analyze` you will find the format parameter defined as such:

format | FormatSpec. Format associated with this split. Formats can be declared via strings ("xx.x") or function. In cases such as analyze calls, they can character vectors or lists of functions.

But as you can see there are no links to other docs or functions that lists useful formats. We should enhance the network of @seealso by adding these info in a common place (I would opt for ?list_formats) that is linked from many {rtables} functions (the one that have format as argument ideally, while list_formats keeps connection to other formatting functions. Final examples (not to have circular deps) should live in a dedicated vignette in {rtables} along with the format handling/precedence

Originally posted by @Melkiades in insightsengineering/formatters#269 (review) ; specifically here.

The new vignette should be part of https://github.com/insightsengineering/rtables/blob/main/vignettes/format_precedence.Rmd

[martincadek]

Linked to insightsengineering/formatters#141