Clarify that this is pre-alpha software

[choldgraf]

Context

For now, we do not want to make any public commitments about the specific structure, spec, features, etc of this repository. We are using it to explore what a myst implementation in js might look like, but there isn't a clear message to others about the "development state" of the repository. This might lead to confusion about what to expect.

Proposal

We should put banners in the mystjs documentation and README that make it clear what state the project is in. I suggest something like:

Note Unstable codebase!
This project is in a pre-alpha state. It is being used to explore a MyST implementation in JavaScript, but will likely change significantly and rapidly. It is being developed by a small team of people on the Executable Books Project, and may make rapid decisions without inclusive discussion. We will update this documentation when the state of the project has stabilized.

Tasks and updates

No response

[rowanc1]

We have added this to the main page of the documentation, and updated a few other places around the docs where there are some issues, especially that @kai-tub ran into with jtex (sorry!).

Closing this as the notice is now live on the site!