feat: Improve error message for missing mkdocs.yml

[Ansters26]

This pull request addresses the issue where a generic "Unknown error" is shown when a user's mkdocs.yml configuration file is missing. The new behavior provides a clear, actionable error message, consistent with how similar errors are handled for Sphinx.

Fixes: #12378

Problem
-When mkdocs.configuration points to a non-existent file, the build fails with a vague "Unknown problem" message.

-This provides a poor user experience as the root cause of the failure is not obvious.

Solution
-The get_final_doctype method in the BaseMkdocs builder (readthedocs/doc_builder/backends/mkdocs.py) has been updated.

-An os.path.exists() check is now performed to verify that the mkdocs.yml file exists before any attempt is made to read it.

-If the file is not found, a UserFileNotFound exception is raised with a clear error message, instructing the user to check for the missing file.

[humitos]

Hi! Thanks for your PR. We are working together in the PR you linked #12378. There is no need to duplicate work.