fix: Custom Secrets - Small formatting updates & added disclaimer

[AlexanderSehr]

Overview/Summary

• Small formatting updates (Title + Images)
• Added disclaimer about notifying the maintainers about new secrets

As part of this Pull Request I have

• [ ] Read the Contribution Guide and ensured this PR is compliant with the guide
• [ ] Checked for duplicate Pull Requests
• [ ] Associated it with relevant GitHub Issues or ADO Work Items (Internal Only)
• [ ] Ensured my code/branch is up-to-date with the latest changes in the main branch
• [ ] Ensured PR tests are passing
• [x] Updated relevant and associated documentation (e.g. Contribution Guide, Docs etc.)

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