Use American spelling consistently throughout documentation

[Copilot]

Documentation used British spelling (e.g., "Serialisation", "serialise", "behaviour", "favour") while filenames and Python code used American spelling (e.g., serialization.md, serialize_lexer). This inconsistency affected 7 docs files.

Changes

• docs/serialization.md — Serialisation → Serialization, serialise/serialised/Serialising → serialize/serialized/Serializing
• docs/api-reference.md — Serialisation → Serialization, Serialise → Serialize
• docs/index.md — Serialisation → Serialization, serialise → serialize
• docs/parser.md — Serialisation → Serialization, serialised → serialized, favour → favor
• docs/changelog.md — serialisation → serialization, analysing → analyzing
• docs/defining-grammar.md — serialisation → serialization
• docs/error-handling.md — behaviour → behavior

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