Implement JSON builder

[wouterj]

Instead of HTML files, Symfony needs .fjson files with the following schema:

array{
    title: string,
    parents: array<int, string>,
    current_page_name: string,
    toc: array<int, array<string, string>>|array{},
    toc_options: array{maxDepth: int, numVisibleItems: int, size: string},
    next: array{title: string, link: string}|null,
    prev: array{title: string, link: string}|null,
    body: string,
}

[wouterj]

integrated in symfony-tools/docs-builder#200

[stof]

I started a review yesterday that I forgot to submit.

[stof]

should it be FjsonRenderer ?

[stof]

or maybe JsonHtmlRenderer to be consistent with the Sphinx name

[wouterj]

The "fun" thing about the phpDocumentor implementation is that the project renderer can be another format than the node renderer. So this class actually doesn't care about the format of the body (except for the page link extensions 🤔 ).

Keeping consistency with the Sphinx name, but decoupling it from Html is exactly how I got to the JsonRenderer name.

Thanks for all your review work btw!