Add contributing / coding-style docs page

[maxnutz]

Summary

• New docs/contributing.md consolidating: project context, code style (Python >= 3.12, type hints, NumPy docstrings, black), the statistics-function/utils.py independence rule, naming convention, fixed function signature, return-format rules, the mapping-file lookup mechanism, testing rules, and the contribution workflow (issue first, branch, PR checklist, assign @maxnutz).
• Added Contributing: contributing.md to mkdocs.yml nav.
• README stays as-is (additive docs page, not a replacement).
• Closes out part 3 of 3 for fill documentation with life and style #82 — once this merges the temporary broken-link warning introduced in Document the Scenario Explorer on the docs landing page #92 (docs/index.md's [Contributing](contributing.md) link) resolves.

Part 3 of 3 for #82.

Test plan

• mkdocs build succeeds, nav renders, no dead internal links from this page
• pixi run test — 201 passed

🤖 Generated with Claude Code

Summary by Sourcery

Document contributing guidelines and coding style and expose the new page in the documentation navigation.

Enhancements:

• Include the Contributing page in the MkDocs navigation to integrate it into the published documentation.

Documentation:

• Add a dedicated Contributing page detailing project context, code style, statistics-function architecture, conventions, testing rules, and contribution workflow.

[sourcery-ai]

Reviewer's Guide

Adds a dedicated contributing guide to the documentation and wires it into MkDocs navigation, clarifying coding style, statistics function architecture and conventions, mapping configuration, testing rules, and the contribution workflow.

File-Level Changes

Change	Details	Files
Add a dedicated contributing documentation page that describes project context, coding style, architecture rules for statistics functions and helpers, mapping-file usage, testing expectations, and the contribution workflow.	Create a new Markdown document describing project context and high-level purpose of the package. Specify Python version, required type hints, NumPy-style docstrings, and black-based formatting requirements. Document independence rules between statistics functions and helpers, fixed function signature and naming convention derived from IAMC variable names. Describe return-type requirements and index structure for statistics functions and their tests. Outline mapping configuration used by the runtime lookup mechanism and the expected developer workflow for issues, branches, tests, and PRs.	docs/contributing.md
Expose the new contributing page in the documentation site navigation.	Add a Contributing entry to the MkDocs navigation configuration under the documentation section. Ensure the previously broken link to the contributing page now resolves by pointing to the new document.	mkdocs.yml

Possibly linked issues

• fill documentation with life and style #82: This PR fulfills the issue’s contributing/coding-style docs part by adding docs/contributing.md and nav entry.

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[sourcery-ai]

Hey - I've left some high level feedback:

• In the statistics-function conventions, the return-format section requires a MultiIndex with location and unit while the testing section requires country and unit; align these terms or explicitly explain the difference to avoid confusion.
• The "Fixed signature" example shows an extra <config: dict> parameter even though the bullets say every function only takes n and aggregate_per_year; rephrase this section to clearly distinguish the mandatory parameters from optional ones with concrete examples.
• The testing section says to add or update tests only when behavior changes, but the contribution workflow step 4 always asks for tests to be added/updated; clarify when tests are expected (e.g., new variables vs refactors) so the guidance is consistent.

Prompt for AI Agents

Please address the comments from this code review:

## Overall Comments
- In the statistics-function conventions, the return-format section requires a MultiIndex with `location` and `unit` while the testing section requires `country` and `unit`; align these terms or explicitly explain the difference to avoid confusion.
- The "Fixed signature" example shows an extra `<config: dict>` parameter even though the bullets say every function only takes `n` and `aggregate_per_year`; rephrase this section to clearly distinguish the mandatory parameters from optional ones with concrete examples.
- The testing section says to add or update tests only when behavior changes, but the contribution workflow step 4 always asks for tests to be added/updated; clarify when tests are expected (e.g., new variables vs refactors) so the guidance is consistent.

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[maxnutz]

@sourcery-ai dismiss