Updated user documentation content

[jessemapel]

Description

Did a bunch of work on the new user documentation. Also cleaned up some ISIS3 stuff to just ISIS.

Related Issue

No issue, doc overhaul

Motivation and Context

This completes the user portion of the new ISIS website docs. The contributor docs are roughed out in this PR and will be completed in a follow up PR.

How Has This Been Tested?

Generated the documentation locally and reviewed that all links, formatting, syntax, and spelling was correct.

Screenshots (if appropriate):

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Documentation change (update to the documentation; no code change)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• I have read and agree to abide by the Code of Conduct
• I have read the CONTRIBUTING document.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have added tests to cover my changes.
• All new and existing tests passed.
• I have added myself to the .zenodo.json document.
• I have added any user impacting changes to the CHANGELOG.md document.

Licensing

This project is mostly composed of free and unencumbered software released into the public domain, and we are unlikely to accept contributions that are not also released into the public domain. Somewhere near the top of each file should have these words:

This work is free and unencumbered software released into the public domain. In jurisdictions that recognize copyright laws, the author or authors of this software dedicate any and all copyright interest in the software to the public domain.

• I dedicate any and all copyright interest in this software to the public domain. I make this dedication for the benefit of the public at large and to the detriment of my heirs and successors. I intend this dedication to be an overt act of relinquishment in perpetuity of all present and future rights to this software under copyright law.

[ladoramkershner]

I was there, so I read all the documentation as we were smithing it and reviewed each page once we finished smithing each section.

[jessemapel]

@ladoramkershner