[Synthetics] Add Kerberos auth docs

[emilioalvap]

Summary

Adds Synthetics documentation for Kerberos auth, following elastic/synthetics#1139.

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• Yes
• No

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🔍 Preview links for changed docs

• solutions/observability/synthetics/kerberos-for-browser-monitors.md

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Elastic Docs Style Checker (Vale)

Summary: 4 warnings, 2 suggestions found

⚠️ Warnings (4): Fix when the suggestion improves clarity or correctness.

File	Line	Rule	Message
solutions/observability/synthetics/kerberos-for-browser-monitors.md	3	Elastic.MappedPages	mapped_pages should only be added or updated in rare scenarios. Talk with your local technical writer before pushing changes to this key.
solutions/observability/synthetics/kerberos-for-browser-monitors.md	16	Elastic.Spelling	'Auhtentication' is a possible misspelling.
solutions/observability/synthetics/kerberos-for-browser-monitors.md	20	Elastic.Latinisms	Latin terms and abbreviations are a common source of confusion. Use 'for example' instead of 'e.g'.
solutions/observability/synthetics/kerberos-for-browser-monitors.md	22	Elastic.Latinisms	Latin terms and abbreviations are a common source of confusion. Use 'for example' instead of 'e.g'.

💡 Suggestions (2): Optional style improvements. Apply when helpful.

File	Line	Rule	Message
solutions/observability/synthetics/kerberos-for-browser-monitors.md	28	Elastic.WordChoice	Consider using 'top-level' instead of 'first-class', unless the term is in the UI.
solutions/observability/synthetics/kerberos-for-browser-monitors.md	28	Elastic.WordChoice	Consider using 'efficiently' instead of 'simply', unless the term is in the UI.

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