Switch references to executablebooks to instead point to jupyterbook

[choldgraf]

We have a few places in mystmd.org and jupyterbook.org that refer to the Executable Books Project (the old home of Jupyter Book). We also describe the Sphinx stack (e.g. myst-parser) as projects that we steward. We should change our documentation to focus on the jupyterbook team/org/project, and mention the Sphinx part of the MyST ecosystem as a project that is maintained totally separately from the jupyterbook project.