📖 Update to mystmd branding

[rowanc1]

This is a follow on task from executablebooks/team-compass#21.

This will eventually allow a consistent install experience across pip and npm.

See #139

[rowanc1]

@choldgraf tagging you in here based on our meeting from last week on this!

The change here is that the download would change from npm install -g myst-cli to npm install -g mystmd. As we talked about last week this allows us to align with eventually installing from conda and PyPI with the same name in the future (#139).

The install process I have also sped up about 17x. 🚀

As a part of this change, this would also rename the git repo to mystmd and then we have alignment across most of our various pages and doesn't turn off users who see the js part and think it isn't for them! This will also allow us to bring back some of the js-specific docs with less confusion.

Let me know if you have any thoughts on this, I would like to get this out before the webinar on Wednesday as we already looking to have quite a few folks signed up there!

[choldgraf]

Go for it. I think that we should double check our explanation in the future to make sure it matches the mental model people bring (eg MyST is a flavor of markdown but it is also a documentation engine?!) bit I think that will be best done after running these ideas by people, getting feedback, and making adjustments.

[rowanc1]

Sounds good - will get this out today!