[8.x](backport #4246) [Observability docs] Change 'beats' namespace to 'elastic-agent'

[mergify]

Elastic Agent Docker images are currently available under both a beats and elastic-agent namespace. For 9.0 only the latter will be available, so this will update the current (8.15) docs.

Rel: elastic/ingest-docs#1314

---

This is an automatic backport of pull request #4246 done by Mergify.

[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.}

[kilfoyle]

@bmorelli25 @alaudazzi I'd love some help with this 8.x backport. It seems to have pulled in 476 more files than expected. :-)

[mergify]

This pull request has not been merged yet. Could you please review and merge it @kilfoyle? 🙏

[bmorelli25]

@bmorelli25 @alaudazzi I'd love some help with this 8.x backport. It seems to have pulled in 476 more files than expected. :-)

Believe it or not, this is expected 😄