[Documentation] Update SELinux docs to include permissive mode

[ShylajaDevadiga]

• Update docs to include selinux permissive mode install and use of INSTALL_K3S_SELINUX_WARN=true when permissive mode is set.
• With INSTALL_K3S_SELINUX_WARN=true, installation of rpms can be skipped in permissive mode.
• Describe --disable-selinux flag

Reference issue: #1371

[stale]

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