Easyadminbundle: Add a "Next" link in Book chapters

[javiereguiluz]

Important: this should be done on 1.x branch and not in master branch.

As discussed on Symfony Slack, the navigation of Book chapters (https://symfony.com/doc/master/bundles/EasyAdminBundle/index.html) could be more friendly if we add a "Next" link at the bottom of each chapter. There's no need to add a "Previous" link because people just click on the "Back" option of their browsers.

The work to do:

• Add "Next" links, but only for the RST files of this dir: https://github.com/EasyCorp/EasyAdminBundle/tree/master/doc/book

To separate this from the normal contents, let's add a line separator (----- in RST). Another question: how should we create these links? There are lots of options:

( ... content ... )

-----

Next chapter: `Title of the next chapter`_

( ... content ... )

-----

Next: `Title of the next chapter`_

( ... content ... )

-----

Keep reading: `Title of the next chapter`_

( ... content ... )

-----

`Next chapter`_

( ... content ... )

-----

Continue to the `Next chapter`_

etc.

---

Please don't work on this until we decide how to create those "Next" links ... and please, share your opinion about which format do you prefer. Thanks!

[kkreft]

:+1: for the first, the most classic one IMO.

[aminebenhariz]

Why not adding one line before the link to introduce the next chapter ?

Next, we're going to play with YAML configuration and learn how to override templates.
Next chapter

[javiereguiluz]

@aminebenhariz because that usually adds "noise". Better be practical and show the links as straightforward as possible.

So, people seem to agree with this format, so let's use it:

( ... content ... )

-----

Next chapter: `Title of the next chapter`_

Any volunteer to add these links? Thanks!

[kkreft]

@javiereguiluz I can do this.

[javiereguiluz]

Fixed by #2146.