feat: add upgrade page (#9)

docs/202601Upgrade.md

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+---
+title: January 2026 upgrade
+---
+# January 2026 upgrade
+
+From the 5th of January to the 15th of January there will be a major upgrade of the data catalog infrastructure (SciCat - [https://discovery.psi.ch](https://discovery.psi.ch) and [https://dacat.psi.ch](https://dacat.psi.ch))
+
+During this period, data archiving and retrieval will not be possible.
+
+## Required changes
+
+After the upgrade, a few changes will be required:
+
+### CLI changes
+
+If you are using pmodules, no changes are required, otherwise:
+
+1. please download the latest released CLI version (>=v3.0.0) by following the [download instructions](https://github.com/paulscherrerinstitute/scicat-cli?tab=readme-ov-file#manual-deployment-and-upgrade).
+2. Modify your ingestion scripts, following the [v3 instructions](https://github.com/paulscherrerinstitute/scicat-cli?tab=readme-ov-file#v3-changes);
+3. or download the [backwards compatible scripts](https://github.com/paulscherrerinstitute/scicat-cli?tab=readme-ov-file#backwards-compatibility-with-v2) (linux only). Please note these scripts will later be discontinued (~ Q2 2026).
+
+### API changes
+
+Some APIs have been updated. This affects you only if you interact directly with the SciCat APIs.
+
+1. You can already find and compare the new API specifications from our [QA environment](https://dacat-qa.psi.ch/explorer).
+2. Authorization update: API authorization now requires passing your token in the Authorization header, prefixed with Bearer, i.e. Bearer <MY_TOKEN>.
+3. Login endpoint: API login is now available at "/auth/login" rather than "users/login".
+
+### Recieving updates notifications
+
+To receive notifications on updates, please subscribe to the [scicat mailing list](https://psilists.ethz.ch/sympa/info/scicat).
+
+If you have questions, don't hesitate to contact us at [scicat-help@lists.psi.ch](mailto:scicat-help@lists.psi.ch).

docs/index.md

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 ---
 title: Home
 ---
+# :warning: Planned January 2026 upgrade :warning:
+
+Please note that there will be a major upgrade that requires client changes. For details, please refer to the [upgrade page](./202601Upgrade.md).
+
 # PSI Data Catalog Documentation
 
 The PSI Data Catalog is the preferred way to archive and publish research data from the