[PNI] Setup categories navigation snippet

[jhonatan-lopes]

Description

The buyers guide on PNI includes a sub navigation with categories:

The way that this is setup currently will include all categories defined in the CMS that are not flagged with a hidden property and displays them sorted by their sort_order. This allows editors to build the navigation menu with their desired items and order them:

However, this setup is inefficient as there is an additional mental load on editors to keep track of which categories have been hidden and their relative order. It makes more sense instead to build a navigation menu out of the current categories and then order them.

This is the goal of this PR: to replace the current implementation by a "PNI categories navbar" snippet, located in the PNI side menu, that will allow editors to build a categories navigation -- displaying only the items that they want and setting them up in the correct order.

Link to sample test page:
Related PRs/issues: #11770

Checklist

Tests

• Is the code I'm adding covered by tests?

Changes in Models:

• Did I update or add new fake data?
• Did I squash my migration?
• [ ] Are my changes backward-compatible. If not, did I schedule a deploy with the rest of the team? ⚠️ Preboot will need to be disabled! ⚠️

Documentation:

• Is my code documented?
• Did I update the READMEs or wagtail documentation?

Merge Method
💡❗Remember to use squash merge when merging non-feature branches into main

[danielfmiranda]

Amazing work @jhonatan-lopes! LGTM 👍