Update NGF doc set title to align with other doc sets

[travisamartin]

Proposed changes

This PR:

• This PR updates the NGINX Gateway Fabric doc set title to align with the style used in other doc sets.

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• I have read the contributing guidelines
• I have signed the F5 Contributor License Agreement (CLA)
• I have ensured that documentation content adheres to the style guide
• If the change involves potentially sensitive changes, I have assessed the possible impact
• If applicable, I have added tests that prove my fix is effective or that my feature works
• If applicable, I have checked that any relevant tests pass after adding my changes
• I have updated any relevant documentation (README.md and CHANGELOG.md
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Potentially sensitive changes include anything involving code, personally identify information (PII), live URLs or significant amounts of new or revised documentation.

Please refer to our style guide for guidance about placeholder content.

[github-actions]

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