docs: fix broken links across README files

## Description

Several English and Chinese README pairs contain local links that currently resolve to nonexistent paths on `main` (`6752ff8da9b1ea85c8e91b27b0a64fef099f22f9`). A repository-wide Markdown link check found 20 broken prose-link occurrences across 16 README files:

- root and devops security email links omit the `mailto:` scheme (4 occurrences)
- ADK backend license links resolve to the nonexistent `adk/LICENSE` instead of the root license (4)
- OpenAI and OpenSearch2 example lists link to nonexistent `examples/timeout` and `examples/knn` directories (4)
- ACL Langfuse links resolve to nonexistent `libs/callbacks/langfuse` (4)
- ACL OpenAI links resolve to nonexistent `libs/components/...` paths (4)

The missing timeout example has never existed in the target directory history. The OpenSearch2 KNN example was superseded by the existing approximate-search example, which is already linked separately.

The copied Markdown test fixture under `components/document/transformer/splitter/recursive/examples/testdata` is intentionally excluded.

## Expected behavior

Every prose link in maintained README files should resolve to an existing repository path or a valid email URL, with English and Chinese copies kept in sync.

## Suggested fix

Correct the relative paths and email schemes, and remove the two stale example-list entries. Re-run a link checker across all tracked Markdown files.

Search the 16 maintained English and Chinese README files for the five reported link patterns, while excluding components/document/transformer/splitter/recursive/examples/testdata. Correct email schemes and relative paths, remove the stale timeout and KNN entries, keep language copies synchronized, then rerun a link checker across tracked Markdown files and confirm every prose link resolves.

Written by the indexing model from the issue text.