Add constants.rs docs and update CLI's doc header

src/constants.rs

@@ -1,4 +1,10 @@
+//! Constants used throughout the Payphone project.
+
+/// The default command prefix used by the bot for new servers.
 pub const DEFAULT_PREFIX: &str = "p.";
 
+/// A cream white color extracted from Payphone's avatar picture, used for generic embeds.
 pub const COLOR_WHITE: u32 = 0xE2D9D0;
+
+/// A red color used for error messages and moderator embeds.
 pub const COLOR_RED: u32 = 0xFF8787;