[Security Solution] [Investigation Guides] Use new timeline with arrow icon

[kqualters-elastic]

Summary

This pr is meant to be merged after elastic/eui#6731 is merged, released, and said release is used in Kibana. Updates the investigation/insight markdown plugin and button to use a new "timelineArrow" icon, instead of the placeholder currently in use.

Checklist

• This renders correctly on smaller devices using a responsive layout. (You can test this in your browser)
• This was checked for cross-browser compatibility

[kibana-ci]

💚 Build Succeeded

• Buildkite Build
• Commit: 643ad90

Metrics [docs]

Async chunks

Total size of all lazy-loaded chunks that will be downloaded as the user navigates the app

id	before	after	diff
securitySolution	9.1MB	9.1MB	+108.0B

Unknown metric groups

ESLint disabled line counts

id	before	after	diff
enterpriseSearch	17	19	+2
securitySolution	397	400	+3
total			+5

Total ESLint disabled count

id	before	after	diff
enterpriseSearch	18	20	+2
securitySolution	477	480	+3
total			+5

To update your PR or re-run it, just comment with:
@elasticmachine merge upstream