[book] Updated link to Translatable Extension

[a-ast]

Q	A
Doc fix?	yes
New docs?	no
Applies to	2.3+
Fixed tickets	n/a

Since l3pp4rd project was moved it is better to provide link to the new project. Moreover, I think it is better to show the exact page of Translatable Extension instead of a list of extensions.

[javiereguiluz]

A quick win and a nice improvement! @AAstakhov thanks for helping us improve the Symfony docs.

[xabbuh]

Thanks for the heads up @AAstakhov. However, I'd say let's link to the rendered docs on http://atlantic18.github.io/DoctrineExtensions/doc/translatable.html.

[a-ast]

@xabbuh, agree

[xabbuh]

👍

[webda2l]

I don't think promote the old (and buggy) Gedmo implementation is a good idea, KNP is simpler and better.

[xabbuh]

@webda2l Would you like to open a new issue for that? We can, of course, discuss to promote another library but we should discuss that apart from this PR which just fixes the link.

[webda2l]

This is just that this PR "Updated link" could become "Remove link", but more people should give their opinion.
But let's merge this PR for now if you want, no problem.

[weaverryan]

Perhaps @docteurklein can tell us about https://github.com/KnpLabs/DoctrineBehaviors and it we should link there instead - I have also heard good things :)

[docteurklein]

I didn't have a good experience with gedmo translation (specifically) extension neither.
It used to denormalize data in a bad way by default (one single big table).

KnpLabs/DoctrineBehaviors tries to stick as much as possible to use ORM associations.
We just add metadata on the fly (via onLoadMetadata event) and add a few model methods via a trait.

I'm not sure about the other behaviors in the same package (softDeletable f.e), but translatable is pretty solid.