Add roadmap documentation for Open Data service

This document outlines the roadmap for the Open Data service, detailing missions and items removed from the roadmap.

Author: mchaamsler

docs/general/roadmap.md

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+---
+sidebar_position: ...
+---
+
+# Roadmap
+This is the current **roadmap** of our Open Data service, which will guide us until the end of 2026.
+- It is iterated on at least a quarterly basis.
+- It allows us to plan for change. It captures intent, not solutions. The further into the future a mission will take place, the more uncertain it is. 
+- Things can be added to or dropped from the roadmap. In the latter case, we document this below under ‘Items removed from the roadmap’, including the date and reason. 
+
+Our roadmap is based on the [long-term **vision** of our service](...). Our **strategy** is delivery.
+
+## Q1: January until March 2026
+
+### Mission 1: Bulk download
+
+<details>
+  <summary>Open Data extracted from the data warehouse is available faster than their measurement frequency.</summary>
+
+  'Extract, transform and load' is optimised.
+  'Author, schedule and monitor workflows' is optimised.
+</details>
+
+<details>
+  <summary>'Spatial climate analyses' are available.</summary>
+
+  ...
+</details>
+
+<details>
+  <summary>...</summary>
+
+  ...
+</details>
+
+### Mission 2: APIs
+
+<details>
+  <summary>...</summary>
+
+  ...
+</details>
+
+### Items removed from the roadmap
+(here comes a table with 'date', 'item' and 'reason of removal')