Bring documentation into place for JOSS

[goerz]

Required improvements in the documentation:

• Copy "Statement of Need" from Prepare JOSS paper #86 text to main page
• Related software: Cf. "State of the Field" in Prepare JOSS paper #86

On the Overview page

• Give a minimal example
• Summarize the mathematics of the GRAPE method, and the numerical scheme that is implemented
• Describe Semi-AD in the context of GRAPE

Some material from the Semi-AD paper can probably be reused for this