Move built-in integrations, such as with Doctrine ORM, to a separate bundles

[Kreyu]

To make the core bundle as small as possible, each integration with optional third party libraries, such as Doctrine ORM or PHPSpreadsheet, should be moved to separate repositories. This will allow for updating and improving each integration without bumping a version of the core (for example, to prevent updating the bundle for projects not using those 3rd party libraries).

[Kreyu]

I'll probably deprecate the integration of PhpSpreadsheet and focus solely on the OpenSpout for now, as it is far better in my use cases and I want to maintain only the one that I am actively using.

If anyone wants to maintain the integration with PhpSpreadsheet in a separate bundle, I can put its documentation into the official docs.

• https://github.com/Kreyu/data-table-doctrine-orm-bundle
• https://github.com/Kreyu/data-table-open-spout-bundle