add merlin6 quick start

docs/merlin6/quick-start-guide/introduction.md

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+# Introduction
+
+## The Merlin local HPC cluster
+
+Historically, the local HPC clusters at PSI were named **Merlin**. Over the years,
+multiple generations of Merlin have been deployed.
+
+At present, the **Merlin local HPC cluster** contains _two_ generations of it:
+
+* the old **Merlin5** cluster (`merlin5` Slurm cluster), and
+* the newest generation **Merlin6**, which is divided in two Slurm clusters:
+  * `merlin6` as the Slurm CPU cluster
+  * `gmerlin6` as the Slurm GPU cluster.
+
+Access to the different Slurm clusters is possible from the [**Merlin login nodes**](/merlin6/interactive.html),
+which can be accessed through the [SSH protocol](/merlin6/interactive.html#ssh-access) or the [NoMachine (NX) service](/merlin6/nomachine.html).
+
+The following image shows the Slurm architecture design for the Merlin5 & Merlin6 (CPU & GPU) clusters:
+
+![Merlin6 Slurm Architecture Design](/images/merlin-slurm-architecture.png)
+
+### Merlin6
+
+Merlin6 is a the official PSI Local HPC cluster for development and
+mission-critical applications that has been built in 2019. It replaces
+the Merlin5 cluster.
+
+Merlin6 is designed to be extensible, so is technically possible to add
+more compute nodes and cluster storage without significant increase of
+the costs of the manpower and the operations.
+
+Merlin6 contains all the main services needed for running cluster, including
+**login nodes**, **storage**, **computing nodes** and other _subservices_,
+connected to the central PSI IT infrastructure.
+
+#### CPU and GPU Slurm clusters
+
+The Merlin6 **computing nodes** are mostly based on **CPU** resources. However,
+it also contains a small amount of **GPU**-based resources, which are mostly used
+by the BIO Division and by Deep Leaning project.
+
+These computational resources are split into **two** different **[Slurm](https://slurm.schedmd.com/overview.html)** clusters:
+
+* The Merlin6 CPU nodes are in a dedicated **[Slurm](https://slurm.schedmd.com/overview.html)** cluster called [**`merlin6`**](/merlin6/slurm-configuration.html).
+  * This is the **default Slurm cluster** configured in the login nodes: any job submitted without the option `--cluster` will be submited to this cluster.
+* The Merlin6 GPU resources are in a dedicated **[Slurm](https://slurm.schedmd.com/overview.html)** cluster called [**`gmerlin6`**](/gmerlin6/slurm-configuration.html).
+  * Users submitting to the **`gmerlin6`** GPU cluster need to specify the option ``--cluster=gmerlin6``.
+
+### Merlin5
+
+The old Slurm **CPU** _Merlin_ cluster is still active and is maintained in a best effort basis.
+
+**Merlin5** only contains **computing nodes** resources in a dedicated **[Slurm](https://slurm.schedmd.com/overview.html)** cluster.
+
+* The Merlin5 CPU cluster is called [**merlin5**](/merlin5/slurm-configuration.html).
+