Drop Markdown output fmt since Hugo defines it

[chalin]

• Contributes to Add support for agent-friendly content generation #2596
• Drops outputFormats.Markdown since Hugo defines it, though Hugo uses the markdown capitalization. (Note that letter case isn't significant, but it's worth knowing the letter case used to define the name.)
• Renames the PRINT output format to print, since it isn't an acronym
• Updates the agent-friendly support plan accordingly
• There is no change to the generated site files other than the timestamp