Add Apertis distribution rebasing guide

Add a guide covering the step by step process of rebasing the Apertis distribution. This is aimed primarily at the core Apertis developers, so adding a new "maintenance" session for such documentation.

Signed-off-by: Martyn Welch martyn.welch@collabora.com

content/maintenance/_index.md

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+date = "2024-04-24"
+weight = 100
+
+title = "Maintenance"
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+
+The documents provided in this section is targeted towards the Apertis maintainers, covering the tasks required to maintain and develop the core Apertis distribution.
+
+For higher level descriptions of the technology employed in Apertis, please see
+the [architecture documents]({{< ref "architecture/_index.md" >}}). For
+documentation targeted at developers and users of Apertis, please see the
+[guides]({{< ref "guides/_index.md" >}}).