add docs for iml backup server check

docs/20_Checks/check_iml_backup_server.md

+# Check IML backup server
+
+## Introduction
+
+This check is for IML Backup server.
+This check executes the script 'storage_helper.sh' to get thebackup status.
+
+see https://os-docs.iml.unibe.ch/iml-backup-server/
+
+## Requirements
+
+There can be a permission issue to reach the storage-helper script or while reading the backup target dirs.
+On our servers the backup target is using a user *imlbackup*. To give access to its resources we added the group "imlbackup" for icinga
+
+```txt
+$ id icingaclient
+uid=1041(icingaclient) gid=1041(icingaclient) groups=1041(icingaclient),1031(imlbackup)
+                                                                             ^
+                                                                             |
+                                additional group for icinga client user -----+
+```
+
+## Syntax
+
+```txt
+______________________________________________________________________
+
+CHECK_IML_BACKUP_SERVER
+v1.0
+
+(c) Institute for Medical Education - University of Bern
+Licence: GNU GPL 3
+
+https://os-docs.iml.unibe.ch/icinga-checks/Checks/check_iml_backup_server.html
+______________________________________________________________________
+
+Show status of backup server.
+You can get the count of connections or the status of server backup.
+
+See https://os-docs.iml.unibe.ch/iml-backup-server/
+
+SYNTAX:
+check_iml_backup_server [-d DIRECTORY] -m MODE
+
+OPTIONS:
+    -h or --help   show this help.
+    -d or --dir    set installation dir of iml deployment to find its check skript
+                   default dir: /opt/imlbackup/server
+                   script: /opt/imlbackup/server/storage_helper.sh
+    -m or --mode   mode: set type of result; one of status|backupstatus
+                     backupstatus  status of backup sets of all servers
+                     status        count of connected servers
+
+EXAMPLE:
+check_iml_backup_server -m status 
+check_iml_backup_server -m backupstatus 
+
+```
+
+## Examples
+
+### Backup server status
+
+``check_http -u https://www.example.com/`` is a check that makes an http GET request.
+The queck is OK if the responded status code is no error - if it is 2xx (OK) or a redirect (3xx).
+
+```txt
+# ./check_iml_backup_server -m backupstatus
+OK: Server Backups - 36 servers in backup - 0 errors - 7 disbled - 0 running
+List of server backups grouped by status (non-OK only)
+
+  server                                        : start                : end                  : duration :  rc : age [h] : size 
+
+D oldserver.example.com                         : 2023-10-17 23:06:29  : 2023-10-17 23:11:05  :      276 :   0 :    1554 : 199G /netshare/restic-backup/oldserver.example.com
+(...) 
+
+  server                                        : start                : end                  : duration :  rc : age [h] : size 
+
+ |notstarted=0;;;0;36 disabled=7;;;0;36 ok=29;;;0;36 running=0;;;0;36 error=0;;;0;36 
+
+```