docs: Funnel chart type has no documentation page

## What's missing

The Funnel chart type (cubedevinc/cubejs-enterprise PR #13506, with a horizontal-orientation follow-up in #14510) is fully shipped but has **no documentation at all**. It's a genuinely new chart type: a stage dimension + measure, tapered shape, conversion-rate labels between stages, and a horizontal orientation option. It's also mirrored into the embed SDK.

Confirmed via a docs-mintlify audit: `docs-mintlify/docs/explore-analyze/charts/chart-types/` has no `funnel.mdx`, and it's absent from that directory's `docs.json` `pages` array (12 entries: area, bar, boxplot, heatmap, html, index, kpi, line, map, pie, sankey, scatter, table). A repo-wide search for "funnel" only turns up an unrelated filter-icon tooltip and the `Funnels` data-modeling recipe (a modeling concept, not this chart type).

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

This is a **new page**, not a small edit — comparable in scope to the existing `sankey.mdx` page (which documents field requirements, cycle/edge-case handling, and grouping options). Per the routine's customer-facing-criteria rules, a feature at this scope goes through planning/ticketing rather than landing as an unreviewed drive-by page.

## Suggested scope

- New page `docs-mintlify/docs/explore-analyze/charts/chart-types/funnel.mdx`, modeled on `sankey.mdx`'s structure: field requirements (stage dimension + measure), conversion-rate label behavior, horizontal vs. vertical orientation, and any edge cases (e.g., single-stage funnels, zero-value stages).
- Add the entry to `chart-types/index.mdx`'s chart list and to `docs.json`'s chart-types `pages` array.
- Confirm whether/how it's exposed in the embed SDK and note that if relevant.

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 reading docs-mintlify/docs/explore-analyze/charts/chart-types/sankey.mdx and the chart list in chart-types/index.mdx. Add the new funnel.mdx page covering the stated field requirements, labels, orientations, and edge cases, then update docs.json and the chart list; done means the page and navigation entries are present and embed SDK exposure is confirmed or documented.

Written by the indexing model from the issue text.