DOC: Example using Vertexselect with source index

[Gnefil]

Reference issue (if any)

#14094

What does this implement/fix?

This enhances the documentation tutorial 20_ui_events.py by providing an example to VertexSelect.

Additional information

Continuation of PR #14126 and #14154.

[Copilot]

Pull request overview

This PR enhances the UI events tutorial by adding a concrete, end-to-end example demonstrating how to react to VertexSelect events using the source_id payload (for indexing source-space–aligned data like forward-model gain matrices). It also includes a small internal adjustment in Brain picking logic related to computing the selected source index.

Changes:

• Add a new tutorial section showing how to subscribe to "vertex_select" and use event.source_id to drive a topomap.
• Adjust volume picking code in mne.viz.Brain to use a source_id variable when determining the selected point.
• Add a changelog fragment for the documentation update.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 3 comments.

File	Description
tutorials/visualization/20_ui_events.py	Adds a VertexSelect usage example that indexes a forward gain matrix via source_id.
mne/viz/_brain/_brain.py	Tweaks how the selected index is computed/named during volume picking before publishing VertexSelect.
doc/changes/dev/14169.other.txt	Adds a changelog entry for the new tutorial example.

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