Move all PrestaShop example modules inside this repository

[matks]

• demo-cqrs-hooks-usage-module: demo example for hooks / v1.7.6 => WIP https://github.com/PrestaShop/example-modules/pull/17
• doctrine: dmo example for Doctrine usage / v? => ?
• customers: demo example for grid hooks / v1.7.5 or v1.7.4 => ?
• crea_clearcache: demo example to add a button in layout/ v? => ?

[matks]

@eternoendless @Quetzacoalt91 Can you confirm the scope of these issues ? Is this all the "demo modules" we have ?

[matks]

Other candidates:

• https://github.com/sarjon/demo-grid-module
• https://github.com/friends-of-presta/customers

[davidglezz]

https://github.com/PrestaShop/paymentexample