docs: update xlang/native serialization doc

[chaokunyang]

Why?

Clarify the split between xlang serialization and same-language native serialization in the Fory guides, and keep website sync plus release-version tooling aligned with the updated docs structure.

What does this PR do?

• Renames language guide pages from cross-language serialization to xlang serialization and updates related links, headings, sidebar positions, and terminology across the runtime guides.
• Adds native serialization guides for Java, Python, C++, Go, and Rust, covering when to use native mode, registration, schema evolution, supported runtime-specific object surfaces, performance guidance, and troubleshooting.
• Adds security configuration guidance for multiple runtime guides, including registration, schema/version checking, depth and size limits, and safer model-shaping recommendations.
• Updates the Java xlang guide to clarify xlang mode configuration and cross-runtime behavior.
• Enables orphan deletion for synced docs directories in .github/sync.yml so renamed guide pages are removed from the site target.
• Fixes Kotlin version bumping in ci/release.py to update the Kotlin parent POM and child module parent versions, with an explicit error when the expected POM marker is missing.

Related issues

AI Contribution Checklist

• Substantial AI assistance was used in this PR: yes / no
• If yes, I included a completed AI Contribution Checklist in this PR description and the required AI Usage Disclosure.
• If yes, my PR description includes the required ai_review summary and screenshot evidence of the final clean AI review results from both fresh reviewers on the current PR diff or current HEAD after the latest code changes.

Does this PR introduce any user-facing change?

Documentation is updated for native and xlang serialization guidance. This PR does not introduce a public API change or binary protocol compatibility change.

• Does this PR introduce any public API change?
• Does this PR introduce any binary protocol compatibility change?

Benchmark

Not applicable; this is documentation and release-script maintenance.

[ayush00git]

@chaokunyang any idea about the v1.0.0 release date?

[chaokunyang]

I will start the release after this pr get merged

[pandalee99]

LGTM！