docs: add Combo Charts visualization page

[jswir]

What

Adds a Combo Charts page to the visualization docs for the new # combo_chart dual-axis chart type introduced in malloydata/malloy#3008 — two measures on two independent y-axes (bars + line), for comparing measures with different units or magnitudes (volume vs. rate, count vs. dollars).

• New src/documentation/visualizations/combo_charts.malloynb: a properties table plus runnable examples using the existing flights sample — smart defaults, explicit channels with swapped marks, the crossover pitfall + axis pinning (y.min/y2.min), line styling (line_width/points), neutral axes (color_axes=false), embedded # y/# y2 tags, and nested-in-table.
• src/table_of_contents.json: entry after Line Charts.

Content mirrors the in-repo renderer_tags_overview.md / renderer_tag_cheatsheet.md updates that ship in malloydata/malloy#3008, adapted into executable notebook examples.

⚠️ Blocked — draft, needs a renderer version bump

The # combo_chart tag is only understood by a @malloydata/render build that includes the plugin from malloydata/malloy#3008. This site currently pins:

"@malloydata/malloy":  "0.0.359",
"@malloydata/render": "0.0.359",

At that version the notebook will not render — the # combo_chart tag falls back / errors. To un-block:

1. feat(render): add combo_chart dual-axis chart type malloy#3008 merges.
2. A @malloydata/render release is cut that includes it. The render package is at 0.0.426 in-repo today (latest published 0.0.425), so the release including #3008 will be 0.0.426+ (whatever the actual release number is when it merges).
3. Bump both @malloydata/malloy and @malloydata/render in package.json (they release in lockstep at the same version) to that release, and refresh the lockfile — ideally as its own commit so the version bump is reviewable separately from the doc page.

Only then does the page render live, and this PR can come out of draft. The Malloy queries in the notebook are valid today; only the # combo_chart rendering depends on the bump.

Test plan

• table_of_contents.json validated as JSON.
• Examples reuse fields already exercised across the visualization docs (dep_delay, distance, dep_time, carriers.nickname) against data/flights.parquet.
• Live-render verification deferred until the renderer bump above lands.