Document CodeChecker skip limitations

[lntutor]

Why:
The skip argument was not documented, so users could reasonably try absolute host paths that cannot match files inside Bazel's sandbox.

What:

• document the CodeChecker skip-file syntax
• explain why workspace-relative patterns are required
• add an example matching the repository's skip tests
• note that current standard and per-file analysis both support skip

Validation:

• git diff --check
• compared the example with test/unit/skip/BUILD
• verified both rule implementations pass skip to CodeChecker

Addresses:
Fixes #193

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