MapScript Documentation Updates

[geographika]

• Re-run constants.py file to add new constants to the API docs pages.
• Add mapscript to requirements as mentioned in suggested improvements on docs for building docs #836, and remove missing function pages.
• Update ReadMe to use PowerShell rather than Windows CMD.

[pvgenuchten]

tested ok for me, with this #836 is resolved

[jmckenna]

thanks @geographika and @pvgenuchten !