docs(features): document multi-account dashboard panels and the Accounts filter

[RamanKharchee]

Description

Documents what shipped for custom dashboards this week, on the Custom Dashboards page:

• the dashboard-level Accounts filter, how it composes with each panel's own picker, and the header chip that names what a panel is showing
• the dashed All accounts line on time series — per label set, gap-tolerant — with a by (...) example
• stat and gauge totals with the hover breakdown and the n of N accounts count for a missing account
• the Account column on multi-account tables
• the per-account series cap and its warning

Eight screenshots from the dev tenant: First Dashborad (the multi-account panels), GitHub Runners (ARC v2) - DND (dashboard overview) and Expensive Queries Stress Test (per-label totals). The dashboard-list image that was a commented-out TODO is now real. Full-page shots are 1568px wide per the repo convention; the panel-level ones are tight crops.

Redaction: the prospect account name that appeared in the first cut was replaced with poc-cluster in every image before capture (page text rewritten in the browser, then re-shot, so the images are clean rather than painted over). No hosts, tokens or emails appear in any image or in the page.

How Has This Been Tested?

Image references resolve (grep img/dashboard against ls doc-server/docs/features/img). No hosts, emails or prospect names in the page or images. Not built locally — the page is Markdown plus PNGs, no MDX components added.

🤖 Generated with Claude Code

[gemini-code-assist]

Code Review

This pull request adds a new section to the custom dashboards documentation detailing how panels function across multiple accounts, including dashboard-wide filtering, visualization behaviors, and series limits. The review feedback suggests several improvements to the documentation's clarity, grammar, and tone, standardizing the capitalization of "K8s", and ensuring confidential prospect names are redacted from the referenced screenshots.

[RamanKharchee]

Review resolution

Finding	Resolution
Prospect name in screenshots	Redacted — all five images re-shot with the name rewritten to poc-cluster before capture
K8S → K8s (3 places)	Fixed — the page now uses K8s everywhere
"reaches only one account"	Fixed — "queries only one account"
"A panel none of the picked accounts belong to"	Fixed — reworded as suggested
"past two"	Fixed — "when three or more are selected"
"a label set only one account reports"	Fixed — added "that"
"assumes the parts add"	Fixed — "assumes the values are additive"

Single amended commit; image references re-checked.