Document library management page

[CalebJ2]

Create documentation for the library management pages. Probably somewhere at https://submitty.org/sysadmin and have a link from the page #93 adds.

[mwilde1998]

A markdown file with text and pictures has been created and pushed to the wwu-cs/submitty.github.io instructor_documentation branch. For now, I put the md in _docs/instructor, because I don't know if we can justify a different tab or folder for admin pages. I'd rather let the core Submitty team figure that out.

[mwilde1998]

I wouldn't consider this finished yet. I would like to wait until dead week when we have the library manager mostly finished before I try updating the documentation.

[mwilde1998]

The documentation page and images have been updated, and I believe its a fairly accurate representation of the final version. I've asked my frontend admin team to review it.