docs: document YAML alias and anchor limitations

[slegarraga]

What & why

Documents how internal/parser handles YAML aliases and anchors so users know what is supported in metadata.yaml. The parser uses gopkg.in/yaml.v3, but it does not expand structural aliases or merge keys, and the guide now says that explicitly.

Closes #15.

Verified behavior

• A mapping with a direct anchor, such as server: &server, parses normally.
• An alias used in place of a mapping returns expected mapping node.
• A merge key such as <<: *base returns expected mapping node.
• A scalar alias inside a leaf value may decode through the YAML library, but Codrao does not rely on it and the guide treats it as outside the supported contract.

Verification

• go test ./... passes.

[SSOHEB]

Great addition! Thanks for your first contribution!