Remarkup is very much extensible, with extensions and applications being able to add their own fun things.
But the current documentation is a single static page in this install (https://we.phorge.it/book/phorge/article/remarkup/).

Build a more fun documentation UI for remarkup, that will run on each install, and allow each extension to bring its own documentation.

Guides is an app that was initially created for a similar use-case, so maybe it can hold this documentation UI, or just some of it be copied over.

Below are some examples of things that aren't documented (or are hard to find):

Fiflet

Standard:

figlet {{{
 hello
}}}

Custom font small:

figlet (font=small) {{{
 hello
}}}

Custom font banner:

Custom font big:

Custom font bubble:

Custom font digital:

Custom font ivrit:

Custom font lean:

Custom font mini:

Custom font mnemonic:

Custom font script:

Custom font smscript:

Custom font shadow:

Custom font smshadow:

Custom font slant:

Custom font smsslant:

Custom font standard:

Custom font term:

Figlet Builtin fonts:

https://we.phorge.it/source/phorge/browse/master/externals/figlet/fonts

Figlet Code Implementation:

https://we.phorge.it/source/phorge/browse/master/src/infrastructure/markup/interpreter/PhabricatorRemarkupFigletBlockInterpreter.php

Cowsay

cowsay {{{
 hello
}}}

Custom eyes, custom think, custom tongue:

cowsay (think=yes,eyes=XX,tongue=^^) {{{
 hello
}}}

Other things from the original Cowsay in Perl as supported as-well:

cowsay (cow=stegosaurus) {{{
 hello
}}}

It seems we support also "companion", from Portal:

cowsay (cow=companion) {{{
 hello
}}}

Cowsay builtin cows:

https://we.phorge.it/source/phorge/browse/master/externals/cowsay/cows/

https://we.phorge.it/source/phorge/browse/master/resources/cows/builtin/

Cowsay custom cows could be placed here:

phorge/resources/cows/custom

Cowsay Code Implementation:

https://we.phorge.it/source/phorge/browse/master/src/infrastructure/markup/interpreter/PhabricatorRemarkupCowsayBlockInterpreter.php

I think it is quite important that these things are mentioned in the documentation. Sometimes people think that Remarkup is a lame version of Markdown, but right now it is pretty obvious that they are two separate worlds and both interesting. I wish I had lived longer knowing these features instead of discovering them by accident.