Adds conftest.py of common fixtures for integration tests

[spjuhel]

This is based on data test used for the integration tests of the static and interpolated trajectories.

The objective is to provide:

• "as comprehensible as possible and as minimalistic as possible data (exposure, hazard, impfset)"
• Factories

Changes proposed in this PR:

• Adds fixtures and factories for minimalistic objects for integration testing
• Adds an quick example of how to use them

We need to decide if this is something we actually want.
Otherwise, I will move the content to the trajectory integration test directly.

PR Author Checklist

• Read the Contribution Guide
• Correct target branch selected (if unsure, select develop)
• Descriptive pull request title added
• Source branch up-to-date with target branch
• Documentation updated
• Tests updated
• Tests passing
• No new linter issues
• Changelog updated

PR Reviewer Checklist

• Read the Contribution Guide
• CLIMADA Reviewer Checklist passed
• Tests passing
• No new linter issues

[spjuhel]

Notes from discussion:

• shift the examples to a (possibly existing) page in the documentation

[spjuhel]

Note for this PR (or rather for future improvements):

• Have also fixtures with the DEMO files
• Plan (a hackathon?) to move tests to use the fixtures

[spjuhel]

I've added a documentation page,
I chose to make a new one because direct integration in the "Writing tests" one did not feel right, but I'm happy to reconsider. Note that there are two mentions/references in the "Writing tests" one!

I tried to find the balance between being explicit and concise (had help from Claude to be honest).

I am wondering about putting actual values in the documentation as this implies that we keep them up to date with the file (although the values should technically not change).

Quick access: https://climada-python--1273.org.readthedocs.build/en/1273/development/Guide_test_fixtures.html