General improvements to the Entity Pdf module code, documentation and functionalities

Ok ... the MR!4 implements the following tech features:

• adoption of README.md file with module documentation
• extended compatibility with Drupal 11
• implementation of an "Entity Pdf Download" action plugin, to automatically generate collated pdf entities, via Views Bulk Ops, directly replicating a similar action present (and very useful) in the Entity Print module (https://git.drupalcode.org/project/entity_print/-/blob/8.x-2.x/src/Plugi...)
• replacements of many \Drupal calls (that should be avoided in classes) in favour of the correct use dependency injection
• perform Drupal and PHP coding standards fixes and full compliance with phpstan quality standards and phpcs quality standards (according to Drupal, DrupalPractice and Slevomat CodingStandards)
• implementation of GitLab CI

and is ready for QA and Peer Review.

Note: Due to the amount of contribution performed on this, I felt to add also myself in the list of maintainers of this Entity Pdf module, in the newly added README.md file. Just feel free to object on that, eventually ...

PS: this MR!4 changes are both major improvements to the Entity PDF code quality and functionalities, so as they (should) stay respecting complete backward compatibility with the actual 2.0.6 module release ...
Anyway all this could be reflected into a brand new 3.0.0 release of the Entity Pdf module (if not a more simple 2.1.0).

Last but not least, a screenshot of the new implementation of an "Entity Pdf Download" action plugin, to automatically generate collated (paginated) pdf entities, via Views Bulk Ops.

Having received Maintainer privileges, I am merging all this into the 2.x branch ... and closing this.
I am going to deploy a new 2.1.0 Entity PDF module version with all this General improvements.

Automatically closed - issue fixed for 2 weeks with no activity.