[navigation-menu][docs] Clarify purpose + focus behaviour, and point disclosure-style nav dropdowns to Popover

Hey there BaseUI team!
I came across a topic that, when improved a bit might genuinely be useful to the next person hitting this. We are using BaseUI for our dropdown components. What we had to do is to implement NavigationDropdown that is using WAI-ARIA **Disclosure Navigation Menu**. First thought was to use your `NavigationMenu` component for that, but that wasn't the choice as I learned on the way.

Your `NavigationMenu` implements the ARIA **menubar / mega-menu** pattern (hover-first, composite arrow-key navigation, and focus moves *into* the content panel on keyboard open). That's a different pattern from the WAI-ARIA **Disclosure Navigation Menu** (a single button that shows/hides a list of links, where **focus stays on the button** and the user Tabs into the links).

The names are easy to confuse, and the docs don't currently distinguish them, so it's easy to pick `NavigationMenu` for a simple "button → dropdown of links" and then be unable to get disclosure behaviour. And that's exactly what I did.

### What the docs currently say

- **NavigationMenu**: only "A collection of links and menus for website navigation." No mention of the intended pattern, the keyboard model, or that **focus moves into the panel on keyboard open** (and that this isn't configurable).
- **Popover**: documents `initialFocus` (incl. `false` to keep focus on the trigger) but has no use-case guidance — nothing indicates it's the right primitive for a disclosure-style navigation dropdown.

### Why it matters

For a disclosure navigation menu, the [WAI-ARIA APG](https://www.w3.org/WAI/ARIA/apg/patterns/disclosure/examples/disclosure-navigation/) specifies that opening the disclosure **keeps focus on the button**, and it deliberately avoids the `menu` role/menubar keyboard model. `NavigationMenu`'s force-focus-into-content behaviour (no opt-out I could find) can't express that, whereas `Popover` + `initialFocus={false}` can.

### Suggested docs improvements

1. **NavigationMenu**: add a short "When to use" note — it's for menubar/mega-menu site navigation; describe the keyboard/focus model (focus enters the panel on keyboard open).
2. Point users who want the **disclosure navigation** pattern (single button revealing a list of links, focus stays on the trigger) to **Popover** with `initialFocus={false}` + a `` landmark and real `` links.
3. Optionally, add "disclosure navigation" as a listed use-case on the **Popover** page maybe? This is actually the only component that I think would do the trick for this use case, but maybe there is something else in the lib that I didn't see. Let me know 🙂

PS: if you accept contributions, then I can, of course, also submit a PR with those changes.

Start with the NavigationMenu and Popover documentation pages linked in the issue, then compare their documented focus behavior with the two WAI-ARIA APG references. Done means the NavigationMenu pattern and focus model are clarified, and disclosure navigation is pointed to Popover with initialFocus={false}, a landmark, and real links.

Written by the indexing model from the issue text.