[DOCS-11797] Kerberos Authentication for Synthetics

[aliciascott]

What does this PR do? What is the motivation?

• New Guide for Kerberos Authentication for Synthetic Monitoring
• Also updates pages through out Synthetic Monitoring Docs and getting started pages to reference this new guide
• Also added a new tab under Authentication protocols and re-ordered them to match the UI

Merge instructions

Merge readiness:

• Ready for merge

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Additional notes

[github-actions]

✅ Documentation Team Review

The documentation team has approved this pull request. Thank you for your contribution!

[github-actions]

Preview links (active after the build_preview check completes)

New or renamed files

• https://docs-staging.datadoghq.com/aliciascott/DOCS-11797-Syn-kerberos/synthetics/guide/kerberos-authentication

Modified Files

• https://docs-staging.datadoghq.com/aliciascott/DOCS-11797-Syn-kerberos/getting_started/synthetics/private_location
• https://docs-staging.datadoghq.com/aliciascott/DOCS-11797-Syn-kerberos/synthetics/browser_tests/app-that-requires-login
• https://docs-staging.datadoghq.com/aliciascott/DOCS-11797-Syn-kerberos/synthetics/guide/
• https://docs-staging.datadoghq.com/aliciascott/DOCS-11797-Syn-kerberos/synthetics/guide/authentication-protocols
• https://docs-staging.datadoghq.com/aliciascott/DOCS-11797-Syn-kerberos/synthetics/platform/private_locations/