Add attribution and source citations for open source release

- Create ATTRIBUTION.md crediting WoWDev Wiki, TrinityCore, MaNGOS,
  and third-party libraries used
- Add WoWDev Wiki format specification links to file format parsers
  (ADT, BLP, DBC loaders)
- Add protocol documentation reference to opcodes.hpp

include/pipeline/dbc_loader.hpp

@@ -14,6 +14,7 @@ namespace pipeline {
  *
  * DBC files store game database tables (spells, items, maps, creatures, etc.)
  * Format: Fixed header + fixed-size records + string block
+ * Format specification: https://wowdev.wiki/DBC
  */
 class DBCFile {
 public: