As an ESL content manager, I want to be able to have a third level of navigation on ESL website documentation.

Business case:
ESL Utils subsection/page under Core is not informative and detailed at that moment.
In purpose of having detailed documentation for esl-utils module section is going to be extended with detailed documentation for each feature. To present such a long content on the site, it was decided to split the section into subcategories ('async', 'decorators', etc)
So it is required from the tech point of view to be able to have the third level of nesting for the page in website navigation.

Sidebar (draft preview):

Category Page:
The accordion was considered for nested third-level pages.

@ala-n , @abarmina
Detailed description TBD