docs: add table of contents to README

[avasis-ai]

Summary

Adds a table of contents to the README for easier navigation of the extensive documentation.

Changes

• Added a ## Table of Contents section after the badges and before "What is TruffleHog"
• Linked all major sections and their subsections using standard Markdown anchor links
• No content changes, purely navigational addition

Closes #2983

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Note

Low Risk
Low risk documentation-only change; no code or behavior is modified.

Overview
Adds a Table of Contents section near the top of README.md, linking to the main documentation headings and key subsections for easier navigation.

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