docs(optimization-guide): add internal table of contents

[israelolrunfemi]

docs(optimization-guide): add internal table of contents (#947)

• Insert ## Table of contents block after the opening H1
• TOC covers all H2 and H3 sections with correct GitHub anchor slugs
• Add regeneration hint comment for future maintainers
• Fix heading hierarchy where levels were skipped (list specifics if any)
• Update cross-links in to match new anchors

Closes: #947
Backlog: I-036

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