Improve documentation how to run tests

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Related Issue(s):

Running only "make test" after a git checkout, like advised in the GitHub pull request template, does not work:

ERROR collecting stac_fastapi/extensions/tests/test_transaction.py __________________________________________________________________________________________
ImportError while importing test module '/home/xxx/projects/stac/stac-fastapi/stac_fastapi/extensions/tests/test_transaction.py'.
Hint: make sure your test modules/packages have valid Python names.
Traceback:
/usr/lib/python3.12/importlib/__init__.py:90: in import_module
    return _bootstrap._gcd_import(name[level:], package, level)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
stac_fastapi/extensions/tests/test_transaction.py:13: in <module>
    from stac_fastapi.extensions.core.transaction.request import (
E   ModuleNotFoundError: No module named 'stac_fastapi.extensions.core.transaction.request'; 'stac_fastapi.extensions.core.transaction' is not a package

Description:

It is not obvious for newcomers that make install must be called before make test. This PR fixes it in README and GitHub pull request template.

PR Checklist:

• pre-commit hooks pass locally
• Tests pass (run make test)
• Documentation has been updated to reflect changes, if applicable, and docs build successfully (run make docs)
• Changes are added to the CHANGELOG.