Add documentation for creating APM agent keys for EDOT SDKs

[alexandra5000]

Summary

This PR adds a dedicated page for creating APM agent keys for EDOT SDKs, with a reusable snippet to reduce content duplication. This change addresses the need for EDOT-specific API key documentation.

Closes #4296

Generative AI disclosure

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1. Did you use a generative AI (GenAI) tool to assist in creating this contribution?

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• No

2. If you answered "Yes" to the previous question, please specify the tool(s) and model(s) used (e.g., Google Gemini, OpenAI ChatGPT-4, etc.).

Tool(s) and model(s) used: Claude 4.5 Sonnet to help draft some sections, make the snippet work as it should, fix build errors, etc.

[github-actions]

Vale Linting Results

Summary: 1 suggestion found

💡 Suggestions (1)

File	Line	Rule	Message
solutions/observability/apm/opentelemetry/create-apm-agent-key-for-edot-sdks.md	65	Elastic.Versions	Use 'or later' instead of 'or higher' when referring to versions.

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[github-actions]

🔍 Preview links for changed docs

• solutions/observability/apm/opentelemetry/create-apm-agent-key-for-edot-sdks.md
• solutions/observability/apm/api-keys.md
• solutions/observability/get-started/quickstart-monitor-your-application-performance.md

[alexandra5000]

Vale Linting Results

Summary: 3 suggestions found

💡 Suggestions (3)

To the future reviewer: I don't think these are good suggestions in the context of this change. LMK if you disagree!

[theletterf]

Good work! Some suggestions.

[mdbirnstiehl]

Overall looks good! I left a few comments, let me know what you think.

[alexandra5000]

Hey @hegerchr - since you were the one to open the issue, would you mind reviewing this? 🙂 Thank you!

[hegerchr]

Looks good to me. Thanks for updating the docs.

[alexandra5000]

@mdbirnstiehl @bmorelli25 - Comments applied! Looks like I still need approval from experience docs - could one of you do it please?