Document testing SSSOM-Py against a development version of the schema.

[gouttegd]

• [x] docs/ have been added/updated if necessary
• [ ] make test has been run locally (no need)
• [ ] tests have been added/updated (not applicable)
• [ ] CHANGELOG.md has been updated. (no need)

This PR updates the CONTRIBUTING.md document to add a section about how to locally run the test suite, and especially how to locally run SSSOM-Py’s test suite, since changes to SSSOM’s LinkML schema can negatively impact the Python implementation.

It also updates the pull request template to add a checkbox to remind contributors that any change to the LinkML schema should be tested against SSSOM-Py (or rather, SSSOM-Py should be tested against the new schema).

[matentzn]

@hrshdhgd Can you also review / approve please? Thank you!