Update from code changes: document metrics catalog embedding

[mintlify]

Summary

Documents the new metrics catalog embed surface so users know how to embed the project metrics catalog and configure Explore/write access on the token.

Changes

• New guide guides/embedding/metrics-catalog.mdx covering when to use it, prerequisites, React SDK usage, and JWT config (including canExplore and writeActions).
• Added Lightdash.MetricsCatalog section to the React SDK reference with props, basic usage, and token generation.
• Added a Metrics catalog token section to the embedding reference and included metricsCatalog in the shared content.type union and "Known limitations" summary.
• Listed the metrics catalog on the "What can you embed?" page (intro, comparison table, next-steps cards) and added it to the embedding nav in docs.json.

Context

Follows upstream lightdash/lightdash#25378 ("feat: metric catalog on embed"), which added the metricsCatalog embed content type, Lightdash.MetricsCatalog SDK component, and content.canExplore support on the new token.

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