[nishantmonu51]

Docs for the custom viz components added in #9850.

• Turns the Custom Viz page into a section under Canvas Widgets with three pages: Custom Viz, Chart Types and Vega-Lite. The first keeps the existing URL.
• Custom Viz covers the gallery, the component editor and test values, params, the query, the chart spec (encodings, field semantics, colors, chartProperties) and adding a custom viz to a canvas.
• Chart Types lists the 32 chart types by gallery family, with their channels and chartProperties from Flint 0.4.1 and a rendered example per family.
• Vega-Lite covers converting a chart spec, what converting writes, the rules and .fields templating for a vega_spec (including the JSON escaping from fix: JSON-escape values templated into a component's vega_spec #10042), and six examples: reference line, target highlight, second axis, running total, theme palette and legend selection.
• Uses the Convert to Vega-Lite wording and the viz/ folder from feat: say "Convert to Vega-Lite" and create custom viz under viz/ #10044.
• Screenshots are from Rill Developer with the OpenRTB example project, and every YAML example was run there.
• Also adds custom_components to the feature flag list, links from the canvas pages and the component reference (schema descriptions, component.md regenerated), and a rule in the developer agent's component instructions to keep vega_spec placeholders inside JSON strings.

Checklist:

• Covered by tests
• Ran it and it works as intended
• Reviewed the diff before requesting a review
• Checked for unhandled edge cases
• Linked the issues it closes
• Checked if the docs need to be updated. If so, create a separate Linear DOCS issue
• Intend to cherry-pick into the release branch
• I'm proud of this work!