docs: add appendix for Operators & Symbols (closes #4931)

[wrvnnull]

Summary

Resolves #4931 by adding an Appendix: Operators & Symbols to the Sway Book reference section.

The appendix is a quick-reference table of arithmetic, comparison, logical, bitwise, and assignment operators plus common symbolic tokens (::, ->, =>, @, .., etc.), complementing the existing Keywords page.

Changes

• docs/book/src/reference/appendix_operators_symbols.md - the appendix
• docs/book/src/reference/index.md - link it
• docs/book/src/SUMMARY.md - register under Sway Reference

Test plan

• mdbook-compatible markdown
• Linked from index + SUMMARY

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[cursor]

PR Summary

Low Risk
Documentation-only changes to the mdbook; no compiler, runtime, or API behavior is affected.

Overview
Adds a new Appendix: Operators & Symbols reference page with tabular quick-reference for arithmetic, comparison, logical, bitwise, and assignment operators, plus symbolic tokens (::, ->, =>, @, ranges, ?, etc.) and compound literals ([], (), {}).

The page is wired into the Sway Book via Sway Reference in SUMMARY.md and a new bullet on reference/index.md, alongside the existing Keywords appendix.

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[wrvnnull]

Re-triggering CLA check after signing the Fuel Labs Contributor License Agreement. Thanks!