docs : Add documentation to use anemoi offline. when using anemoi.utils.grids

[floriankrb]

What maintenance does this project need?

Looking at https://github.com/ecmwf/anemoi-utils/blob/develop/tests/test_grids.py
and https://github.com/ecmwf/anemoi-utils/blob/develop/src/anemoi/utils/grids.py

• The function grids() should be documented.

• And the use of the config should be documented as well.

    `cat ~/.config/anemoi/settings.toml`
  
    ```
    [utils]
    grids_path = "~/path/to/my/npz"
    ```
  

• Support for a list of paths in the config could also be added

  `cat ~/.config/anemoi/settings.toml`
  
  ```
  [utils]
  grids_path = [
                   "~/path/to/my/directory_1",
                   "~/path/to/my/directory_2",
                  ]
  
  ```
  

Organisation

No response

[floriankrb]

We could start a page "How to use anemoi offline without internet access".

Then point to this page from
https://anemoi.readthedocs.io/projects/graphs/en/latest/graphs/node_coordinates/reduced_gaussian.html (where it explains how to create a graph by specifying only the grid name).

We may need to add a anemoi-utils prepare-remote ./local_path, to save locally what is needed :
(This would download the data from wget https://get.ecmwf.int/repository/anemoi/grids/grid-o48.npz)