fix: Revise project disclaimer and add maturity badge (#641)

Updated disclaimer to reflect project maturity status.

***As a contributor to the Anemoi framework, please ensure that your
changes include unit tests, updates to any affected dependencies and
documentation, and have been tested in a parallel setting (i.e., with
multiple GPUs). As a reviewer, you are also responsible for verifying
these aspects and requesting changes if they are not adequately
addressed. For guidelines about those please refer to
https://anemoi.readthedocs.io/en/latest/***

By opening this pull request, I affirm that all authors agree to the
[Contributor License
Agreement.](https://github.com/ecmwf/codex/blob/main/Legal/contributor_license_agreement.md)

Author: mchantry

README.md

@@ -1,9 +1,10 @@
 # anemoi-core
 
-**DISCLAIMER**
-This project is **BETA** and will be **Experimental** for the foreseeable future.
-Interfaces and functionality are likely to change, and the project itself may be scrapped.
-**DO NOT** use this software in any project/software that is operational.
+[![Static Badge](https://github.com/ecmwf/codex/raw/refs/heads/main/Project%20Maturity/incubating_badge.svg)](https://github.com/ecmwf/codex/raw/refs/heads/main/Project%20Maturity#incubating)
+
+
+> \[!IMPORTANT\]
+> This software is **Incubating** and subject to ECMWF's guidelines on [Software Maturity](https://github.com/ecmwf/codex/raw/refs/heads/main/Project%20Maturity).
 
 A mono-repo containing core training and modelling functionality for Anemoi, providing the packages `anemoi-training`, `anemoi-models`, and `anemoi-graphs`.