The `prev` option in fjson files is always null

[javiereguiluz]

We were checking the generated fjson files and we found that next option works perfectly, but the related prev option is always null.

Example:

This file -> https://github.com/EasyCorp/EasyAdminBundle/blob/master/doc/index.rst

The crud item is after dashboards and before design, but in the fjson file we get this:

    "next": {
        "title": "Design",
        "link": "design.html"
    },
    "prev": null,

Now, if you look at design, you should find crud as prev. Instead, you find this:

    "next": {
        "title": "Fields",
        "link": "fields.html"
    },
    "prev": null,

Given that next is working perfectly, maybe this has a non-complex fix? Thanks!