feat: add body signals, empty for handling, and self-closing element support

[mohamedmansour]

Parser:

• Inject body_start/body_end raw signals around content
• Skip empty bodies entirely (no fragment generated)
• Handle void elements by checking parser end_tag presence
• Handle self-closing tags (/> syntax) for SVG and other elements

Docs:

• Remove protocol output sections from user-facing docs (signals, attributes, for, if) — show only template/state/output
• Add body signals and empty for docs
• Add copilot instruction: user docs must not expose protocol internals