Allow inspection / persistence of build log

[fwkoch]

When running myst build in a large project, the resulting log messages are quite verbose and only available in the console. This means (1) errors early in the build may be entirely lost, depending on how many lines of history are maintained, and (2) on revisiting a build, there is no way to look at logs.

There are a couple improvements we could make here:

• An easy first pass would be implementing a --quiet cli option that could make the logs less verbose, only raising errors/warnings. This would help address (1) - still, this relies on console history.
• Another option could be exposing build logs somewhere in the local myst site. Then, similar to how you can browse the site config or the underlying JSON data for each page, you could also browse the build logs. This would address both (1) and (2) but would require implementing some custom local behavior that we do not want on deployed myst sites.