Support MKDOCS_CONFIG_FILE environment variable - #4140

Fixes #3970.

Summary

• allow MKDOCS_CONFIG_FILE to select the config file when --config-file is not provided
• preserve explicit --config-file precedence over the environment variable
• add config file selection tests

Tests

• PYTHONPATH=<downloaded wheels>:. python3.13 -m unittest mkdocs.tests.config.base_tests.ConfigBaseTests.test_load_config_file_from_env mkdocs.tests.config.base_tests.ConfigBaseTests.test_config_file_precedes_env
• direct _open_config_file assertions for env config and default mkdocs.yml fallback
• git diff --check -- mkdocs/config/base.py mkdocs/tests/config/base_tests.py