docs: rework choosing flavors, add boot-modes and secureboot

Signed-off-by: Eike Waldt <waldt@b1-systems.de>
On-behalf-of: SAP <eike.waldt@sap.com>

Author: yeoldegrove

repos-config.json

@@ -6,7 +6,7 @@
       "docs_path": "docs",
       "target_path": "projects/gardenlinux",
       "ref": "docs-ng",
-      "commit": "3d3392ef4f115a2c37d0963200b6086c4e972f3d",
+      "commit": "de42c5b1ff11495de832b321eada207031c64c1f",
       "root_files": [
         "CONTRIBUTING.md",
         "SECURITY.md",