Support showing a list of commands that the bot understands

#1 (closed) added a way to show the current factoids to users, but it would also be nice to show the commands the bot understands on the site as well. Currently this isn't really feasible because the list of commands is hard-coded in the bot, but over in decbot4#15 (comment 2030673272) I've suggested treating commands as a kind of factoid which would make all of them appear in the database.