feat: add opt-in cross-module compatibility warnings

[AlexanderLanin]

Summary

Today, any validation mismatch in an integrated documentation build aborts the build. This is correct and safe, but it prevents integration entirely until every participating module is perfectly aligned.

This feature allows integration to proceed during temporary version skew, but only for explicitly enabled, well-defined cross-module compatibility cases. It does not suppress arbitrary validation failures: local findings and every non-enabled category remain fatal.

Enabled compatibility findings stay highly visible: they are emitted in the build output, shown prominently on the generated documentation start page, and written to JSON and HTML reports for CI follow-up.

Supported opt-in cases

• missing mandatory attributes in mounted or imported documentation modules
• missing mandatory links in mounted or imported documentation modules
• version mismatches in need links when at least one endpoint is external

Refs #674

[github-actions]

License Check Results

🚀 The license check job ran with the Bazel command:

bazel run --lockfile_mode=error //src:license-check

Status: ⚠️ Needs Review

Click to expand output

[License Check Output]
Extracting Bazel installation...
Starting local Bazel server (8.6.0) and connecting to it...
INFO: Invocation ID: 8831b025-0539-4d75-92d7-4d88d9ea5ace
Computing main repo mapping: 
Computing main repo mapping: 
Loading: 
Loading: 0 packages loaded
Loading: 0 packages loaded
Loading: 0 packages loaded
    currently loading: src
WARNING: Target pattern parsing failed.
ERROR: Skipping '//src:license-check': no such target '//src:license-check': target 'license-check' not declared in package 'src' defined by /home/runner/work/docs-as-code/docs-as-code/src/BUILD
ERROR: no such target '//src:license-check': target 'license-check' not declared in package 'src' defined by /home/runner/work/docs-as-code/docs-as-code/src/BUILD
INFO: Elapsed time: 6.032s
INFO: 0 processes.
ERROR: Build did NOT complete successfully
ERROR: Build failed. Not running target

[github-actions]

The created documentation from the pull request is available at: docu-html