Allow single-file builds for ipynb/md sources with no myst.yml file, without any warnings

[fperez]

When running

myst build  <filename.md/ipynb>

Myst currently produces this warning:

⚠️  myst.yml Loading project from path with no config file: <path>
Consider running "myst init --project" in that directory

Proposal

This warning is unnecessary - running explicit builds to export to PDF or html from a source file that has all necessary info contained in the frontmatter should be supported without warnings. It's a legitimate use case and should run cleanly without any warning (other than those caused by issues in the source file).