Use proper naming and spelling of things

[stevepiercy]

• Use "iCalendar" when describing a file format that follows that standard.
• Use "jCal" when describing a file format that follows that standard.
• Use ".ics" or ".ifb" when describing a file's extension. See https://datatracker.ietf.org/doc/html/rfc5545.html#section-8.1.
• Don't use "ICS" anywhere, including the tabbed interface. It's not used as an acronym in RFC 5545. Use "iCalendar" instead.

This could also be an issue in icalendar.

[niccokunzmann]

I worked on it and think this can be closed.
Also, this now uses pydata's theme.