Add class to generate redirects.json from redirects.xml sidecar

[jordikroon]

The goal for this PR is to migrate all doc related redirects out of web-php.
Instead we generate a redirects.json from redirects.xml which is then loaded in-place.

Redirects not related to docs are NOT migrated. However shortcuts are.

[derickr]

I don't really have to say much, but feel free to merge this. We can then see if it "breaks" anything, before merging doc-en (and web-php).

[alfsb]

The Redirects class really generates data, or only validates it, while changing from XML to JSON?

Also, collectXmlIds() collects all xml:id, but I do not think all IDs become filenames of chunked manuals. AFAIK, the chunk logic is calculated using Docbook tag names, and this changes by render format.

So, if the purpose of xml:id collection is to verify if a redirect target exists, this can only be determined by inspecting the rendered chunked output directory for filenames (not IDs), for the specific rendering used by PHP Manual.