UI/UX (sidebar + navigation) is a core lever in “productifying” a service. Done right, it orients users, reduces friction, and lets your product feel polished. Below is how I’d formalize the navigation spec, and then some open-source tool suggestions you can adopt.

Navigation in SaaS / Documentation:

Below is a crisp definition of what your target navigation should achieve and a recommended hierarchy/behavior model.

Objectives / Principles

1. Findability & Context
   Users should always know where they are (breadcrumb, highlight) and what else is in that section. Navigation must mirror the mental model of the audience.

2. Progressive Disclosure
   Rather than overwhelming with full depth, collapse at higher levels (e.g., only show H1 headings). Expand subtrees only when user drills in.

3. Persistent Sidebar + In-Page Sync
   The sidebar stays fixed (or sticky) on desktop; as the user scrolls the main content, the sidebar updates (highlighting the current section). The in-page headings serve as the anchor navigation.

4. Responsive Behavior
   On mobile/tablet, the sidebar should become a drawer, slide/in off-canvas, or collapse to minimal icons. Must preserve full navigation without breaking.

5. Media/Inline Resources in Navigation
   For pages containing video or rich media, indicate in the navigation (e.g. an icon or “▶️ Video” sub-link) so the user can jump. Also keep scroll sync even across non-text elements.

6. Versioning / Contextual Filters
   If your product has multiple modules / versions / features, the nav should permit switching context (e.g. “Docs for Module A / Module B”) so that the user only sees relevant subtrees.

7. Breadcrumb + Back Links
   Even if the sidebar is strong, users often benefit from a breadcrumb above content or “Up / Parent section” links.

8. Lazy Load / Virtualization for Large Trees
   If your documentation or product has many nodes (hundreds), you may need to lazy-render subtrees to avoid performance drag.

Suggested Hierarchy / Behavior Model

Here’s a pattern that mirrors what you like (Notion, Odoo, MS Word navigation):

Sidebar  
├── Module / Section A  (collapsed)  
│    ├── Subsection A.1  
│    │     ├── Topic A.1.a  
│    │     └── Topic A.1.b  
│    └── Subsection A.2  
├── Module / Section B  
│    ├── Overview  
│    ├── How To  
│    └── Reference  
└── Module / Section C  

(Main content area—on scroll, internal headings sync to expand/collapse nav)  

• At page load, only top-level modules / sections are expanded at H1 level.

• When a user navigates to a module, that subtree expands; others remain collapsed.

• As user scrolls inside a module page, the nav further drills down to H2 / H3 as needed (but doesn’t expand siblings).

• Clicking a nav item scrolls or navigates to that section; clicking again toggles collapse if it has children.

• The video or rich media sections are assigned a nav node (e.g. “▶ Tutorial Video”) and are scrolled into view when clicked.

• On narrow screens, the sidebar becomes a drawer toggled by a hamburger icon; inside the drawer, the same behavior applies.

Essentially: “Collapse at H1; expand on demand; sync with scroll” + consistent statefulness across navigation.

Open-source / frameworks / libraries you can adopt

Here are tools or frameworks that already support this kind of navigation or can be adapted easily. You’ll need to integrate them with your backend or static site setup.

Additionally:

• The “Navgoco” jQuery plugin has been traditionally used for accordion/expand collapse nav behavior in docs sites. I'd Rather Be Writing

• Many static documentation frameworks (Sphinx, Docusaurus, Hugo, etc.) have sidebar / TOC plugins or themes that support collapse + scroll sync.