Support underscores for configuration file (aka, _myst.yml)

[choldgraf]

We currently support a configuration file called myst.yml. When MyST sites start to have a lot of pages, it can be hard to quickly find the myst.yml file among the long list. You have to scroll down to the ms and find it there.

For users that find this annoying, we could support one or more versions of the myst.yml filename. For example, either _ or . would bump it to the top of the list of files in most file explorers, making it easier to quickly find.

Note: this might become unnecessary once we implement nested file/folders for documents:

• Support folder hierarchies and nested files in URLs without flattening them #670

So if this would be non-trivial to support then we could just prioritize #670 instead