2.9.1rc6: Bug fixes and feature improvements

What's Changed

• [2.9] Update hello-world requirements for NVFlare 2.9.1 by @holgerroth in #5507
• [2.9] Fix documentation build with Sphinx 8 by @IsaacYangSLA in #5510
• [2.9] Refresh unified confidential computing documentation by @nvidianz in #5504
• [2.9] Refresh confidential computing overview by @chesterxgchen in #5527
• [2.9] Restore Azure confidential computing overview by @chesterxgchen in #5530
• [2.9] Fix admin connector inspection reply decoding by @nvidianz in #5567
• [2.9] Simplify CoCo workflow for existing clusters and Trustee by @IsaacYangSLA in #5557
• [2.9] Fix async gRPC startup URL construction by @nvidianz in #5633
• [2.9] Add beginner CoCo prerequisite appendix by @IsaacYangSLA in #5635
• [2.9] Simplify confidential deployment and CVM Builder workflows by @nvidianz in #5582
• [2.9] Accept expiry_warning_seconds in CVM vault project config by @emma-hu123 in #5660
• [2.9] Fix FedAvg partial checkpoints and update CC diagnostics and architecture by @nvidianz in #5665
• [2.9] Restore job-log uploads after failed deletion by @nvidianz in #5663
• [2.9] Fix CVM Builder deployment workflows and diagnostics by @nvidianz in #5667
• [2.9] Fix Trustee provenance and CVM bootstrap phase diagnostics by @nvidianz in #5673

New Contributors

• @emma-hu123 made their first contribution in #5660

Full Changelog: 2.9.1rc5...2.9.1rc6