[doc](file-cache) add file_cache_info system table

[freemandealer]

Summary

• add English and Chinese documentation for information_schema.file_cache_info to the 4.x and dev docs
• document block-level cache fields, filtering and aggregation examples, and query overhead considerations
• clarify that the system table is available starting from Doris 4.1 and is not supported in Doris 4.0.x
• add the page to the 4.x and dev sidebars and link it from the File Cache monitoring section

Validation

• ran git diff --check
• validated sidebar JSON, front matter, Markdown fences, admonitions, and relative link targets
• started Docusaurus for English and Chinese locales
• verified the 4.x and dev pages in a headless browser, including the version note and OFFSET / REMOTE_PATH columns

Notes

The Docusaurus development build reports pre-existing warnings in unrelated documentation. No warning or build error points to the files added by this change.