Pelican config tweaks to make category use easier

Minor configuration modifications to make categories not show up in the main
menu of the site. This keeps the pages as first class entities. I may add categories to the sidebar at some point in the future to make them browsable.

Change the default category to "none" because I want to use pelican's normal default, "misc" for things I intentionally mark as miscelaneous, not things I forget to categorize.

Retain git data in the output directory so my gitignore works and keeps me from seeing generated files in git status.
Ignore all files in output; they're generated automatically