docs: add CI/CD & Testing Infrastructure report for v3.0.0

[tkykenmt]

Summary

This PR adds documentation for CI/CD & Testing Infrastructure improvements in OpenSearch v3.0.0.

Reports Created

• Release report: docs/releases/v3.0.0/features/multi-plugin/ci-cd-testing-infrastructure.md
• Feature report: docs/features/multi-plugin/ci-cd-testing-infrastructure.md

Key Changes Documented

k-NN Plugin:

• New GitHub Action for remote index builder integration tests
• Sanity checks for radial search, filtering search, and KNN search

SQL Plugin:

• Calcite integration test framework with standalone and remote modes
• New configuration options: plugins.calcite.enabled, plugins.calcite.fallback.allowed
• Test optimization: unified Calcite IT with NonFallbackCalcite IT
• Dependency fixes: bouncycastle version inline, shadow JAR fix

Related PRs

• k-NN [v3.3.2] Dependency Bumps #2620: GitHub Action for remote index builder ITs
• sql [feature] Date Format #3342: Build Calcite integration test framework
• sql [feature] PPL Commands (Calcite) #3413: Separate with/without pushdown ITs
• sql [bugfix] k-NN Bugfixes #3447: Use entire shadow jar to fix IT
• sql [feature] Merged Segment Warmer #3469: Set bouncycastle version inline
• sql [dev] Define file naming conventions for feature reports #3544: Only enable fallback for tests that need it

Closes #187