R1:32b5139d46dc

Diviner: Improve documentation for remarkup code blocks

Summary:
Improve remarkup reference documentation.

* Added a heading for `Code Blocks`
* Added a sentence mentioning the ##```lang## syntax which was added in {T15481}

Test Plan:
* Run `bin/diviner generate` to regen diviner docs.
* Check out the code blocks section of [[/book/phorge/article/remarkup/#code-blocks|Remarkup Reference]]

Reviewers: valerio.bozzolan, O1 Blessed Committers

Reviewed By: valerio.bozzolan, O1 Blessed Committers…