[docs](file-cache) Document index-only write policies

[bobhan1]

Summary

• document the global enable_file_cache_write_index_file_only policy, including its scope, precedence, expected behavior, configuration example, and operational trade-offs
• document the Base and Cumulative Compaction-specific index-only policies and clarify that they refine existing cache-write decisions
• add the related index and Segment footer/index profile metrics
• update the File Cache internals page to keep the Compaction behavior description consistent

Related Doris PRs

• [Enhancement](file-cache) Add fine-grained control for compaction file cache doris#60609
• [feature](cloud) Support file cache write index only doris#64995

Scope

This PR updates the current and 4.x File Cache documentation in English, Chinese, and Japanese. Japanese source pages are updated under ja-source.

Validation

• git diff --check
• reviewed the documented configuration names, precedence, and behavior against the current Doris master implementation and the related PR descriptions
• verified that the added current and 4.x content is aligned across all three languages
• Docusaurus build not run, following the request to keep this round limited to documentation content

Versions

• dev
• 4.x
• 3.x
• 2.1 or older (not covered by version/language sync gate)

Languages

• Chinese
• English
• Japanese candidate translation needed

Docs Checklist

• Checked by AI
• Test Cases Built
• Updated required version and language counterparts, or explained why not
• If only one language changed, confirmed whether source/translation counterparts need sync