Write a migration guide from Tact 1.x to Tact 2.0

[novusnota]

Links for inspiration:

• https://www.swift.org/migration/documentation/migrationguide/
• https://docs.python.org/3/howto/pyporting.html
• https://gleam.run/cheatsheets/gleam-for-elixir-users/
• https://gleam.run/cheatsheets/gleam-for-erlang-users/