Adding custom error collection info to the ET FE standalone docs

[meghanlo]

What does this PR do? What is the motivation?

Merge instructions

• [ ] Please merge after reviewing

Additional notes

[github-actions[bot]]

Preview links (active after the build_preview check completes)

New or renamed files

• https://docs-staging.datadoghq.com/meghanlo/update_fe_standalone_docs/error_tracking/standalone_frontend/collecting_browser_errors

Modified Files

• https://docs-staging.datadoghq.com/meghanlo/update_fe_standalone_docs/error_tracking/standalone_frontend/
• https://docs-staging.datadoghq.com/meghanlo/update_fe_standalone_docs/real_user_monitoring/browser/collecting_browser_errors
• https://docs-staging.datadoghq.com/meghanlo/update_fe_standalone_docs/real_user_monitoring/error_tracking/browser

[estherk15]

Docs editorial review: https://datadoghq.atlassian.net/browse/DOCS-8506