doctrine/rst-parser should be replaced with phpdocumentor/guides

[stof]

Doctrine itself does not use the doctrine/rst-parser package anymore, in favor of phpdocumentor/guides which is actively maintained by the phpdocumentor team, much more spec compliant and more extensible. The Doctrine team plans to abandon the doctrine/rst-parser package (focusing the team resource on maintaining the projects that are in the scope of Doctrine: persistence-related things).

The Symfony docs-builder should migrate to the new package, and will probably be able to remove lots of code added for workarounds of parser issues.

[wouterj]

While preparing for the stable release of phpDocumentor/guides, and with the help of typo3, we've already made huge steps into making this work: https://github.com/wouterj/symfony-docs-guides We need to revive that repository, integrate it with symfony.com and do lots of regression testing. I was already planning on quickly discussing this at symfonycon with Fabien, Javier and Jaap.

When Doctrine migrated their docs to phpDocumentor, I had a chat with Gregoire about this. We didn't expect Doctrine to want to maintain the package any longer and suggested to move its code into this repository as long as symfony.com isn't transitioned yet. But given the low maintenance (there is no change in the past 2 years basically), Gregoire and me decided to keep it in Doctrine for that moment.

So if the only thing blocking Doctrine from abandoning the package is us, let's move the code into this repository.