[Security]: Edit highlighted fields in alert flyout (#2279)

Resolves #1235.
Adds the appropriate 9.1 'applies' label to the tip about
adding/removing custom highlighted fields.

This PR also removes all images on the page since they don't add
significant value to the docs and cost a lot to maintain.

Related PRs:
* #1540
* elastic/security-docs#6924

Author: natasha-moore-elastic