docs(asset hierarchy): describe the rebuilt hierarchy page

[blakeaowens]

Description

The Pro Asset Hierarchy page has been rebuilt, and this guide still describes the version before it. Rewrites the "Visualizing Assets - Hierarchy" section of docs/content/asset_modelling/PRO_hierarchy/asset_hierarchy.md to match the page as it now behaves, and replaces its four screenshots.

What the guide said, and what is now true:

• It described a table you switched away from in order to see the diagram. The Asset list and the diagram are now one page, so selecting Assets in the list draws them immediately. The list can be collapsed to give the diagram the whole page.
• It described five icon buttons inside each Asset node, listed by emoji. Those actions now live in a panel beside the diagram and have names: Open Asset, Edit Asset, Add Child, Change Parent, and Remove From Hierarchy. Each opens beside the diagram instead of in a dialog over it, and an action you cannot use is greyed out with the reason.
• It referred to a "Load More" button. Loading more of the hierarchy is now two named controls, Load Parents (above a node whose parent is not loaded) and Load (below a node with more children than are shown), so they are documented under their own heading.
• The zoom controls moved from the top left of the diagram to the bottom left.

It also documents two things the page could always do but the guide never mentioned: the node colours distinguish the Assets you selected from the ones the diagram loaded around them, and the same side panel chooses which details each node displays.

Screenshots replaced: asset_hierarchy.png, asset_hierarchy_diagram.png, asset_hierarchy_node.png, assets_loadmore.png. The diagram image is shared with the Pro features overview page, which describes the diagram only in general terms and needs no text change.

Based on bugfix because the UI change it documents ships in the same patch release, so the two stay together.

Test results

Documentation only: no code, no tests. The prose was checked against the running page, and every control it names was exercised there.

Documentation

This PR is the documentation change. Translations of this page (.de, .es, .fr, .ja) are left for the usual translation pass; they carry the same replaced images.

Checklist

• Rebased against the latest bugfix.
• Submitted against bugfix, matching the release the documented change ships in.
• Meaningful PR name for the release notes.
• No code changes, so Ruff and Python compliance are not applicable.
• Documentation included (this is the documentation).