design doc for 'Always using _disableInitializers() in constructors'

[AmadiMichael]

This design doc proposes always using _disableInitializers() in proxied contracts constructors rather than the prevalent approach of calling the initialize() function from the constructor.

Issue: https://github.com/ethereum-optimism/optimism/issues/12634

[AmadiMichael]

Discussed in #150. Conclusion was to implement it alongside adding the following semgrep rules:

• Checks that all initialize functions have external and initializer modifiers.
• Checks that all proxied contracts have a constructor and that it calls _disableInitializers()