Improve SELinux docs

[margamanterola]

Current situation

Our SELinux documentation is not relevant in our current state. It mentions steps to enable SELinux Enforcing for rkt containers, which is deprecated. It's also unclear what works and doesn't work right now.

Impact
Our documentation is outdated and confusing, leading to user issues when trying to enable SELinux.

Ideal future situation
We need to do a full review of the SELinux documentation:

• Document how to use SELinux with docker containers.
• Document how SELinux works in Flatcar
• Document how to disable SELinux completely in case it causes issues.