docs: Dashboard comments feature has no documentation

## What's missing

Dashboard comments (cubedevinc/cubejs-enterprise PR #14040, permissions/admin-toggle follow-up in #14273) shipped a threaded (one-level-deep) commenting feature on dashboards: create/edit/resolve/reply, soft delete, comments survive a republish, and there's a per-surface admin on/off toggle. **None of this is documented anywhere in docs-mintlify.**

Confirmed via audit: no page under `docs/explore-analyze/dashboards/`, `docs/explore-analyze/dashboards/widgets/`, or the embedding iframe docs mentions comments. The only trace in the whole docs-mintlify tree is auto-generated API-reference content — `api-reference/changelog.mdx` (v0.5.0, 2026-08-30) notes `EmbedSettings` gained `enableDashboardComments`, and the matching schema exists in `api-reference/api.yaml` — but that's an OpenAPI-derived field description, not a written explanation of the feature for end users or admins.

## Why this needs a ticket instead of a docs PR

This is a new, multi-part feature (the commenting UI/UX itself, the resolve/reply flow, and the admin on/off toggle plus its interaction with embedding) rather than a one-paragraph addition — it likely needs its own section (or page) under Dashboards, plus a mention of the admin toggle in the embedding customization docs.

## Suggested scope

- New "Comments" section under `docs-mintlify/docs/explore-analyze/dashboards/` (or its own page if it's substantial enough) covering: how to add/edit/resolve/reply to a comment, one-level threading, soft delete, and persistence across republish.
- A note in `embedding/iframe/customization.mdx` or `embedding/iframe/dashboards.mdx` about the `enableDashboardComments` admin toggle for embedded dashboards.

Filed as a GitHub issue rather than a Linear ticket because the Linear MCP connector isn't authorized in this session — recommend re-filing in Linear (or linking this issue there) once available.

Start by reviewing the existing pages under docs-mintlify/docs/explore-analyze/dashboards/ and the embedding/iframe/customization.mdx or embedding/iframe/dashboards.mdx pages, then check api-reference/changelog.mdx and api-reference/api.yaml for the existing toggle terminology. Done means end-user documentation covers creating, editing, resolving, replying, one-level threading, soft deletion, republish persistence, and the enableDashboardComments admin toggle.

Written by the indexing model from the issue text.