docs(graphs): update graph docs to template (docathon)

[VeraChristina]

Description

• Update the graph documentation to the anemoi template following the docathon meta discussion.
• Replace the contributing guidelines with reference to main contributing guidelines.
• Fix link to installing section in anemoi training docs to point to installing section in graphs instead.

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📚 Documentation preview 📚: https://anemoi-graphs--219.org.readthedocs.build/en/219/

[VeraChristina]

@b8raoult Would you add the logo with subtitle for graphs?

[aaron-hopkinson]

Thanks for taking this on. Looks good to me (aside from one minor concern re: "command line usage" vs "tool")

Another minor suggestion, could you fix the broken link on L15-16 of index.rst so that it points to the index of the main docs page with intersphinx, please?

I think this should be:

of the packages within the :ref:`anemoi framework <anemoi-docs:index>`.

(Can't do an inline suggestion because it's too far away from a change...)

[aaron-hopkinson]

Looks good - thank you!