Mermaid graphs don't render in documentation

[RodrigoVillar]

Going to the documentation for the persist_worker module, we see:

There's some crates we can use for this:

https://crates.io/crates/aquamarine: straightfoward (just one macro) but doesn't work on docstrings
https://crates.io/crates/simple-mermaid/0.2.0: works on docstrings (I believe) but mermaid graph has to live in a separate file

[demosdemon]

We do use aquamarine, but it requires decorating the diagram with the appropriate attribute. Example:

firewood/storage/src/checker/mod.rs

Lines 771 to 782 in 94641ec

	/// Generate a test trie with the following structure:
	///
	#[cfg_attr(doc, aquamarine::aquamarine)]
	/// ```mermaid
	/// graph TD
	/// Root["Root Node<br/>partial_path: [2]<br/>children: [0] -> Branch"]
	/// Branch["Branch Node<br/>partial_path: [3]<br/>path: 0x203<br/>children: [1] -> Leaf"]
	/// Leaf["Leaf Node<br/>partial_path: [4, 5]<br/>path: 0x2031454545...45 (32 bytes)<br/>value: [6, 7, 8]"]
	///
	/// Root -->|"nibble 0"| Branch
	/// Branch -->|"nibble 1"| Leaf
	/// ```

[demosdemon]

to be fair, that is the only example in the code and it doesn't render because the it's buried deep in a test build. cargo doc doesn't render test docs by default.