sql: make global indexes a new doc and add more contents

[hfxsd]

Introduced a new 'global-indexes.md' document detailing global indexes, their use cases, limitations, and implementation. Updated references and links across multiple files to point to the new documentation, replacing inlined or outdated content. Removed the inlined global index section from 'partitioned-table.md' and adjusted related documentation and best practices to reference the new page. Added supporting images for global index mechanisms and comparisons.

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