docs: state field length limits in the publishing quickstart

[beeswaxpat]

The server.json schema enforces length limits on free-form fields (in the 2025-12-11 schema: description and title 100 characters, name 200, version strings 255), but none of the user-facing docs state them; about.mdx only mentions "strict character limits" without numbers. Hitting the description limit is a common first-publish failure: mcp-publisher init generates the field, the publisher writes a sentence or two, and publish/validate fails with a schema error that has to be traced back to the length. #1184 asks for exactly this documentation.

This adds one note to the quickstart at the point where the publisher first edits server.json, so the limits are visible before the first publish attempt. Complements #1267, which improves the validator's error message for the same failure.

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