Consolidate connection config into shared reference page

[mtoy-googly-moogly]

Summary

• Add new config.malloynb page documenting the shared malloy-config.json format, connection types, setup SQL, and environment variables
• Remove duplicated connection type tables, setup SQL docs, and env var sections from the extension page — replaced with a cross-reference to the new config page
• Update CLI page with current malloy-cli connections command syntax and cross-reference to config page
• Simplify CLI overview page to link to setup docs instead of inlining setup instructions
• Add "Configuration" entry to the table of contents

Test plan

• Verify the new Configuration page renders correctly and all internal links resolve
• Confirm extension and CLI pages link correctly to the new config page
• Check table of contents shows the new Configuration entry in the right position

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