Add Roihu timeline (#2601)

* Add Roihu timeline

* add few links

Author: rkronberg

docs/computing/index.md

@@ -1,8 +1,11 @@
 # Overview
 
 !!! Note
-    For an overview of the LUMI supercomputer, see [the LUMI
-    documentation](https://docs.lumi-supercomputer.eu/hardware/).
+    For an overview of CSC's next national supercomputer Roihu coming in 2026,
+    [see this page](systems-roihu.md).
+
+    For an overview of the LUMI supercomputer, see
+    [the LUMI documentation](https://docs.lumi-supercomputer.eu/hardware/).
 
 Puhti and Mahti are CSC's supercomputers. Puhti has been available for CSC users
 since 2 September 2019 and Mahti has been available since 26 August 2020. LUMI is
@@ -17,6 +20,9 @@ since 2021 (Mahti AI) with Nvidia Ampere GPUs. See [specifications](available-sy
 for details on the systems and [this page for an outline of differences between LUMI-C and
 Mahti](lumi-vs-mahti.md).
 
+Puhti and Mahti will be replaced in 2026 by CSC's next national supercomputer
+called **Roihu**. [Read more about Roihu here](systems-roihu.md).
+
 CSC supercomputers use the Linux operating system and we recommend that you are familiar with
 basics of [Linux command line usage](../support/tutorials/env-guide/index.md) before starting.
 

docs/computing/systems-roihu.md

@@ -4,6 +4,16 @@
     This page contains preliminary information about CSC's next national
     supercomputer Roihu, which is projected to be in researchers' use early
     2026. Please note that the details may evolve over time.
+    [See tentative schedule below](#schedule).
+
+## Schedule
+
+```mermaid
+graph LR;
+    A{{Roihu will be installed at the **end of 2025**}}-->B;
+    B{{Four-week pilot phase in **February 2026**}}-->C;
+    C{{Roihu available for all users in **March 2026**}}-->D{{Puhti & Mahti end-of-life **later during 2026**}};
+```
 
 ## Compute
 
@@ -18,12 +28,12 @@ which supports the AVX-512 vector instruction set. 414 of the CPU nodes will
 have 768 GiB of memory, while the remaining 72 nodes will have an extended
 memory of 1 536 GiB each.
 
-Each GPU node will be equipped with 4 Nvidia GH200 Grace Hopper superchips. 
-Each GH200 superchip comprises one Hopper GPU and one Grace CPU with 
-72 ARM CPU cores which are connected with a very fast interface. Each 
-GH200 superchip has 120 GiB CPU memory and 96 GiB GPU memory, providing 
-a total of 480 GiB CPU memory per node. This gives a total of 528 GPUs and 
-38 016 CPU cores in the whole GPU partition. 
+Each GPU node will be equipped with 4 Nvidia GH200 Grace Hopper superchips.
+Each GH200 superchip comprises one Hopper GPU and one Grace CPU with
+72 ARM CPU cores which are connected with a very fast interface. Each
+GH200 superchip has 120 GiB CPU memory and 96 GiB GPU memory, providing
+a total of 480 GiB CPU memory per node. This gives a total of 528 GPUs and
+38 016 CPU cores in the whole GPU partition.
 
 The system will also provide four visualization nodes with two Nvidia L40 GPUs
 each, as well as four high-memory CPU nodes with 3 TiB memory and higher