Add OTel for RUM docs

[theletterf]

Summary

Fixes https://github.com/elastic/opentelemetry-dev/issues/1160 by creating a guide to send RUM data to Elastic Observability using OpenTelemetry SDKs and Collectors.

Generative AI disclosure

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

• Yes
• No

[github-actions]

Vale Linting Results

Summary: 17 suggestions found

💡 Suggestions (17)

File	Line	Rule	Message
solutions/observability/applications/otel-rum.md	35	Elastic.Acronyms	'CORS' has no definition.
solutions/observability/applications/otel-rum.md	49	Elastic.Acronyms	'NGINX' has no definition.
solutions/observability/applications/otel-rum.md	51	Elastic.Acronyms	'NGINX' has no definition.
solutions/observability/applications/otel-rum.md	85	Elastic.Acronyms	'CORS' has no definition.
solutions/observability/applications/otel-rum.md	121	Elastic.Acronyms	'NGINX' has no definition.
solutions/observability/applications/otel-rum.md	123	Elastic.Acronyms	'NGINX' has no definition.
solutions/observability/applications/otel-rum.md	677	Elastic.Semicolons	Use semicolons judiciously.
solutions/observability/applications/otel-rum.md	701	Elastic.Capitalization	'Content Security Policy' should use sentence-style capitalization.
solutions/observability/applications/otel-rum.md	709	Elastic.Capitalization	'Cross-Origin Resource Sharing (CORS)' should use sentence-style capitalization.
solutions/observability/applications/otel-rum.md	728	Elastic.WordChoice	Consider using 'refer to (if it's a document), view (if it's a UI element)' instead of 'see', unless the term is in the UI.
solutions/observability/applications/otel-rum.md	732	Elastic.WordChoice	Consider using 'can, might' instead of 'may', unless the term is in the UI.
solutions/observability/applications/otel-rum.md	734	Elastic.WordChoice	Consider using 'can, might' instead of 'may', unless the term is in the UI.
solutions/observability/applications/otel-rum.md	771	Elastic.Acronyms	'NGINX' has no definition.
solutions/observability/applications/otel-rum.md	826	Elastic.WordChoice	Consider using 'refer to (if it's a document), view (if it's a UI element)' instead of 'see', unless the term is in the UI.
solutions/observability/applications/otel-rum.md	849	Elastic.Ellipses	In general, don't use an ellipsis.
solutions/observability/applications/otel-rum.md	849	Elastic.Ellipses	In general, don't use an ellipsis.
solutions/observability/applications/otel-rum.md	935	Elastic.WordChoice	Consider using 'can, might' instead of 'may', unless the term is in the UI.

[github-actions]

🔍 Preview links for changed docs

• solutions/observability/applications/otel-rum.md

[david-luna]

This config is proxying the Authorization header from the browser most likely in the form ApiKey .... I have my doubts if we want the key being configured in the web app where is public.

[theletterf]

@david-luna What's the alternative?