Dangling links JupyterLab-myst's web page

[nthiery]

Just reporting two dangling links in JupyterLab-myst's web page. I would have proposed a PR, but I wasn't sure about the appropriate targets for the links.

1. Usage section:

   [MyST][myst-quickstart] is a flavor of ...
   

2. Trust section:

   Jupyter Notebooks implement a [trust-based security model](https://jupyter-notebook.readthedocs.io/en/stable/security.html)
   

   This link should presumably point to https://jupyterlab.readthedocs.io/en/latest/user/notebook.html#trust instead?