Reference implementation for introducer url/obfvsintro

We need a single place/single source of truth how introducer url (invite code) works

• Documentation: Why do we need it? How is it used? How is it implemented? Describe parameters for each implementation like obvs4
• Reference implementation: create invite url based on existing/known parameters, also have a reference decode function
• A place where to save docs/reference implementation
  • does not need to be in bitmask-core
• A provider needs a tool that creates invite codes that can be shared