Add Yard, and begin some documentation

Use Yard to generate documentation. Included in the `.yardopts` file is
configuration to also publish the markdown files in the `wiki/`
directory.

wiki/
- addons.md: Information on writing new addons.
- database.md: Information on using the database and migrations

db/migrate/*: Rename in accordance with documentation

Rakefile: Add the Yard Rake tasks

db/migrate/0100_create_custom_commands.rb

@@ -0,0 +1,14 @@
+class CreateCustomCommands < ActiveRecord::Migration[6.1]
+  def up
+    create_table :custom_commands do |table|
+      table.string :roomid
+      table.string :command
+      table.string :response
+      table.timestamps
+    end
+  end
+
+  def down
+    drop_table :custom_commands
+  end
+end