DISA STIG book removal (#3080)

* content not available for rocky 10
* remove entry from `.pages`
* git rm content from books

Author: sspencerwire

docs/books/.pages

@@ -7,6 +7,5 @@ nav:
     - Learning Rsync: learning_rsync
     - LXD Server: lxd_server
     - Incus Server: incus_server
-    - DISA STIG: disa_stig
     - Sed, Awk & Grep: sed_awk_grep
     - ...