docs(clickstack): document heatmap dashboard tile

[alex-fedotyev]

Closes #6146.

Summary

Adds a Heatmap step to dashboards/index.md so the Custom Dashboards walkthrough covers the new visualization type alongside Line/Bar, and cross-links from the existing Event Deltas page so users discovering the feature in either place find the other.

The Heatmap tile shipped to ClickStack/HyperDX in hyperdxio/hyperdx#2107.

What the new section covers

• When to use a heatmap vs a Line chart: see the shape of a distribution over time, not just the average or a single percentile. Reveals bimodal duration patterns, slow-tail clusters, sudden spreads.
• Source-type constraint: the data source dropdown only shows sources whose source type is Traces. Logs, metrics, and session sources are filtered out, since heatmaps need the source's Duration Expression for span duration. (Matches allowedSourceKinds={[SourceKind.Trace]} in ChartEditorControls.tsx.)
• Pre-fill behaviour once a source is selected: Value set to (Duration)/1e6, Count set to count(), with numberFormat for proper Y-axis labels.
• Using Where to scope the heatmap to a specific service or set of operations, and adjusting the time range to match the period of interest. Wider ranges expose distribution shifts that shorter windows hide.
• The Display Settings drawer (Scale / Value / Count expression). Cross-links to the existing settings table on the Event Deltas page rather than duplicating it.
• The two-query model (bounds + bucket) visible under Generated SQL.
• Drill down to Event Deltas: clicking a cell on a rendered tile reveals a View in Event Deltas action that opens the Event Deltas view with the tile's data source, Where clause, and time range carried over.

Use cases covered

• Building a service-level latency dashboard tile and recognizing a bimodal distribution.
• Pivoting from a tile cell into Event Deltas to investigate which spans drive a slow tail.
• Configuring scale / value / count when the defaults are not enough.

Screenshots

Three screenshots, all dark theme to match neighbouring sections, captured at play-clickstack.clickhouse.com against the demo traces dataset:

• heatmap-tile-editor.png: Heatmap tab selected in the chart editor with ServiceName:"payment" filter and the bimodal preview, showing the Display Settings button.
• heatmap-tile-rendered.png: Saved tile on the dashboard over a 24h window with the fast and slow paths clearly separated.
• heatmap-tile-drilldown.png: cell click revealing the View in Event Deltas action.

Validation

Walked the flow end to end on play-clickstack.clickhouse.com:

• Editor source dropdown returns only ClickPy Traces and Demo Traces when Heatmap is selected; logs/metrics sources do not appear.
• Pre-fill: Value=(Duration)/1e6, Count=count() populate on source selection.
• Cell click on the rendered tile opens the View in Event Deltas action; selecting it lands on /search?source=...&where=ServiceName%3A%22payment%22&from=...&to=...&mode=delta with the data source, Where clause, and time range carried over, and the Event Deltas analysis mode panel selected.

[vercel]

@alex-fedotyev is attempting to deploy a commit to the ClickHouse Team on Vercel.

A member of the Team first needs to authorize it.

[CLAassistant]

Thank you for your submission! We really appreciate it. Like many open source projects, we ask that you sign our Contributor License Agreement before we can accept your contribution.
_{You have signed the CLA already but the status is still pending? Let us recheck it.}

[alex-fedotyev]

Re-opened as #6151 from an in-repo branch so Vercel previews authorize automatically. Same content; broken link event-deltas#customize -> event_deltas#customize fixed there.