Add the OTel quickstart

[mdbirnstiehl]

Description

This PR adds docs for the OTel quickstart to the o11y docs.

Documentation sets edited in this PR

Check all that apply.

• Stateful (docs/en/observability/*)
• Serverless (docs/en/serverless/*)
• Integrations Developer Guide (docs/en/integrations/*)
• None of the above

Related issue

Closes #250

[github-actions]

A documentation preview will be available soon.

• 🔨 Buildkite builds
• 📚 HTML diff
• 📙 Preview page

Request a new doc build by commenting

• Rebuild this PR: run docs-build
• Rebuild this PR and all Elastic docs: run docs-build rebuild

_{run docs-build is much faster than run docs-build rebuild. A rebuild should only be needed in rare situations.}

_{If your PR continues to fail for an unknown reason, the doc build pipeline may be broken. Elastic employees can check the pipeline status here.}

[akhileshpok]

The configuration of OTel collectors in the values.yaml file will be updated in 8.18. The doc should reflect the latest configuration. Please verify the latest configuration with @rogercoll. Thanks

[mdbirnstiehl]

The configuration of OTel collectors in the values.yaml file will be updated in 8.18. The doc should reflect the latest configuration. Please verify the latest configuration with @rogercoll. Thanks

Hi @akhileshpok, isn't the values file only related to the K8s quickstart?