Rollup merge of #141709 - aDotInTheVoid:split-for-docs, r=GuillaumeGomez

jsondocck: Refactor directive handling

Best reviewed commit by commit.

1. Moves directive handling into its own file. This makes it easier to link to in the dev-guide (#2422 (comment)), but also makes the code nicer in it's own right
2. Renames command to directive. This is what compiletest uses, and it's nice to not have 2 words for this.

r? ``@GuillaumeGomez``

Author: matthiaskrgr