[SIEM][Endpoint]Update in-app links to documentation

[benskelker]

There are in-app links to the SIEM docs. For 7.9, the URLs need to be updated as we are combining the SIEM and Endpoint documentation for the unified Elastic Security solution. The new documentation resides in this repo: https://github.com/elastic/security-docs.

When the structure of the 7.9 docs is finalised, I'll open PRs to update the in-app links (including prebuilt rule metadata).

cc @spong @XavierM @randomuserid @MikePaquette @jmikell821 @Donnater

[benskelker]

PR: #71403