Document the number-tile color picker on the HyperDX dashboards page

Follow-up to hyperdxio/hyperdx#2265 (number-tile static color picker).

The number tile in HyperDX dashboards now supports a static color choice from the curated chart palette. The current dashboards page does not mention the option; we should document it once the matching external API parity lands so customers see the same authoring surface in both the UI and dashboards-as-code.

Scope

• Add a section on docs/use/dashboards.md (HyperDX) that documents the number-tile color option in the Display Settings drawer.
• Cover both the categorical tokens (chart-1 through chart-10) and the semantic tokens (chart-success, chart-warning, chart-error), and the rationale (palette tokens reflow correctly across light, dark, and IDE themes).
• Include light + dark screenshots of a number tile with a non-default color.
• Once external API parity lands (tracked at hyperdxio/hyperdx#2359), add an example payload showing the field on a POST /api/v2/dashboards body.

Acceptance

• Customers can find "how do I color a number tile?" through the dashboards page.
• The token list and theme-reflow behavior are documented.
• Light + dark screenshots are paired.
• API example matches the schema in the external API parity PR.

Relates to hyperdxio/hyperdx#1360.