Auto collapse categories when new one is selected (#4003)

## What are you changing in this pull request and why?

Adding a feature that auto-collapses a category on the sidebar when a
new category at the same level is opened.

## Checklist
- [x] Review the [Content style
guide](https://github.com/dbt-labs/docs.getdbt.com/blob/current/contributing/content-style-guide.md)
and [About
versioning](https://github.com/dbt-labs/docs.getdbt.com/blob/current/contributing/single-sourcing-content.md#adding-a-new-version)
so my content adheres to these guidelines.
- [x] Add a checklist item for anything that needs to happen before this
PR is merged, such as "needs technical review" or "change base branch."

website/docusaurus.config.js

@@ -51,6 +51,7 @@ var siteSettings = {
     docs:{
       sidebar: {
         hideable: true,
+        autoCollapseCategories: true,
       },
     },
     image: "/img/avatar.png",