ISD-1656 Add resize storage documentation (#125)

docs/how-to/resize-jenkins-storage.md

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+# How to resize the jenkins-home storage volume
+The default size of the jenkins-home storage volume for a fresh installation is 1GB. While this works for most scenarios, operators might need to have more storage for installing plugins, storing artifacts, and runninng builds/checking out SCMs on the built-in node.
+
+A low disk-space on the built-in node will cause the node to go offline, blocking jenkins from running jobs.
+
+## Create a backup
+From [Backing-up/Restoring Jenkins](https://www.jenkins.io/doc/book/system-administration/backing-up/), This script backs up the most essential files as mentioned in the article:
+* The `master.key` file.
+* Job-related files in the `./jobs`, `./builds` and `./workspace` folders.
+* Plugins (`.hpi` and `.jpi` files) in the `./plugins` folder
+
+```bash
+#!/bin/bash
+export JENKINS_HOME=/var/lib/jenkins
+export JENKINS_BACKUP=/mnt/backup
+
+echo "running backup as $(whoami) in $(pwd)"
+mkdir -p $JENKINS_BACKUP
+cp $JENKINS_HOME/secrets/master.key $JENKINS_BACKUP
+cp -r $JENKINS_HOME/*.xml $JENKINS_BACKUP
+cp -r $JENKINS_HOME/jobs $JENKINS_BACKUP
+cp -r $JENKINS_HOME/builds $JENKINS_BACKUP
+cp -r $JENKINS_HOME/workspace $JENKINS_BACKUP
+mkdir -p $JENKINS_BACKUP/plugins
+cp -r $JENKINS_HOME/plugins/*.hpi $JENKINS_BACKUP/plugins
+cp -r $JENKINS_HOME/plugins/*.jpi $JENKINS_BACKUP/plugins
+
+chown -R 2000:2000 $JENKINS_BACKUP
+tar zcvf jenkins_backup.tar.gz --directory=/mnt backup
+```
+1. Transfer the backup script above to the running unit of the Jenkins-k8s charm and run it
+```bash
+juju scp --container jenkins ./backup.sh jenkins-k8s/0:/backup.sh
+juju ssh  --container jenkins jenkins-k8s/0 /bin/bash
+bash /backup.sh
+```
+2. Retrieve the compressed backup file
+```bash
+juju scp --container jenkins jenkins-k8s/0:/backup/jenkins_backup.tar.gz jenkins_backup.tar.gz
+```
+3. With the data backed-up, we can remove the jenkins-k8s application.
+```bash
+juju remove-application jenkins-k8s
+```
+
+## Restore the backup on a new charm instance
+1. When the application has been deleted, create a new application with the `--storage` flag. In this example we'll deploy the charm with a storage of 10GB
+```bash
+juju deploy jenkins-k8s --storage jenkins-home=10GB
+```
+2. Wait for the charm to be ready, then restore the backup on the new unit.
+```bash
+juju scp --container jenkins ./jenkins_backup.tar.gz jenkins-k8s/0:/jenkins_backup.tar.gz
+tar zxvf jenkins_backup.tar.gz
+chown -R 2000:2000 /backup
+cp -R /backup/* /var/lib/jenkins
+rm -rf /backup /jenkins_backup.tar.gz
+```
+3. Finally restart pebble
+```bash
+pebble restart jenkins
+```