add documentation for e4s/21.05 Cori

@sleak-lbl this is the e4s/21.05 user docs. I have requested review from E4S team. If you can also take a look that would be great.

This is the user documentation for e4s/21.05.

Screenshots of rendered page

Submitters and reviewers please make sure

• internal links are relative and to the .md file
• avoid links named "here"
• alt-text for images is descriptive
• doesn't duplicate existing content
• spell check
• renders correctly
• does not include any identifiable information (real usernames, project names, etc)