Allow users to parse and include documents in their MyST build that aren't part of the table of contents

[agoose77]

It can be useful to have pages that define content (executable outputs, markdown snippets, etc) that are solely meant for re-use in a book, and aren't part of the book itself.

Right now, for a document's content to be available in a book (e.g. with {embed}) it must also be listed in the toc: and will show up in the sidebar.

Instead, there should be a way to get MyST to parse content even though it isn't meant for inclusion in the sidebar.

References

• Sphinx uses orphan annotations in the frontmatter for this, so that files are parsed but not flagged for warnings because they aren't in the TOC
• Original discussion thread

[choldgraf]

@agoose77 I cleaned up the issue body a bit so that it was clearer what kind of end-functionality we needed from a user's perspective