Add Housekeeping docs

doc/11-Housekeeping.md

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+# <a id="Datbase Housekeeping"></a>Database Housekeeping
+
+Your database may grow over time and contain some outdated information. Icinga Certificate Monitoring provides you
+the ability to clean up these outdated info in an easy way.
+
+## Certificates and Targets
+
+The default `cleanup` action removes targets whose last scan is older than a certain date/time and certificates that
+are no longer used.
+
+By default, any targets whose last scan is older than `1 month` are removed. The last scan information is always updated
+when scanning a target, regardless of whether a successful connection is made or not. Therefore, targets that have been
+decommissioned or are no longer part of a job configuration are removed after the specified period. Any certificates
+that are no longer used are also removed. This can either be because the associated target has been removed or because
+it is presenting a new certificate chain.
+
+### Usage
+
+This scan command can be used like any other Icinga Web cli operations like this: `icingacli x509 cleanup [OPTIONS]`
+
+**Options:**
+
+```
+--since-last-scan=<datetime>    Clean up targets whose last scan is older than the specified date/time,
+                                which can also be an English textual datetime description like "2 days".
+                                Defaults to "1 month".
+```
+
+#### Example
+
+Remove any targets that have not been scanned for at least two months and any certificates that are no longer used.
+```
+icingacli x509 cleanup --since-last-scan="2 months"
+```
+
+## Job Activities
+
+The `cleanup` command provides also an additional option with which you can remove jobs activities older than a
+certain date/time.
+
+Jobs activities are usually just some stats about the job runs performed by the scheduler or/and manually
+executed using the [scan](04-Scanning.md#scan-command) and/or [jobs](04-Scanning.md#scheduling-jobs) command.
+By default, all jobs and schedules activities older than `1 month` are removed.
+
+### Usage
+
+This scan command can be used like any other Icinga Web cli operations like this: `icingacli x509 cleanup activities [OPTIONS]`
+
+**Options:**
+
+```
+--job=<name>                Remove all activities matching the given job name.
+--older-than=<datetime>     Remove all jobs activities older than the specified date/time, which can also be
+                            an English textual datetime description like "2 days". Defaults to "1 month".
+```
+
+#### Example
+
+Remove all activities older than two months that belongs to job `lan`.
+```
+icingacli x509 cleanup activities --job lan --older-than="2 months"
+```