Add missing PHP constraint docs

[jonaslagoni]

Reason/Context

In the other languages, we have explicit documentation explaining the constraint rules such as for TypeScript: https://modelina.org/docs/constraints/typescript

However, we are missing one for PHP.

TODO:

• Go over the constraint logic for PHP for constants: https://github.com/asyncapi/modelina/blob/master/src/generators/php/constrainer/ConstantConstrainer.ts
• Go over the constraint logic for PHP for enum value and key constraints: https://github.com/asyncapi/modelina/blob/master/src/generators/php/constrainer/EnumConstrainer.ts
• Go over the constraint logic for PHP for model naming constraints: https://github.com/asyncapi/modelina/blob/master/src/generators/php/constrainer/ModelNameConstrainer.ts
• Go over the constraint logic for PHP for property key constraints: https://github.com/asyncapi/modelina/blob/master/src/generators/php/constrainer/PropertyKeyConstrainer.ts

You could potentially copy-paste the TypeScript one: https://github.com/asyncapi/modelina/blob/master/docs/constraints/TypeScript.md and adapt it to PHP