chore(doc-gen): show arguments as a subsection of the usage section

Previously, on the docs of directives which include the `animation` section, `arguments` are shown as an `h3` element below the `animation` `h2` element, making it look like it's a subsection of `animations`.

This commit ensures that the àrgument` `h3`element is rendered correctly after the `usage` `h2` element.

Fixes #15645
Closes #15646