feat: add developer instructions

[Oisin-M]

Description

Time has come to add some centralised developer instructions to the main earthkit packages that each of the subpackages can link to. Idea is to have as much as possible in the central instructions and as little as possible subpackage-specific instructions so it's more seamless to contribute across the stack and additionally also hopefully even more seamless experience when using the different components.

In this PR, added:

• documentation guidelines (diataxis)
• contributing guidelines
• guidelines for which package to put functionality in
• guidelines for naming (long clear function names, short arguments etc.)
• functional-first design (probably most debatable - needs discussion)
• maximum interoperability with other Python libraries + earthkit packages
• preference for one ground truth implementation over multiple
• guidelines on dependencies (minimal for components, most functionality out of the box for toplevel)
• missing value handling guidance

Contributor Declaration

By opening this pull request, I affirm the following:

• All authors agree to the Contributor License Agreement.
• The code follows the project's coding standards.
• I have performed self-review and added comments where needed.
• I have added or updated tests to verify that my changes are effective and functional.
• I have run all existing tests and confirmed they pass.

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[EddyCMWF]

Approved from a technical implementation and general structure point of view.