Add nextcloud app renovate config for inclusion

We currently have this as renovate config for our nextcloud repo:

{
    "$schema": "https://docs.renovatebot.com/renovate-schema.json",
    "extends": [
        "local>stackspin/renovate-config"
    ],
    "prConcurrentLimit": 2,
    "regexManagers": [
        {
            "fileMatch": [
                "(^|/)values\\.yaml*$"
            ],
            "matchStrings": [
                "github_repository: (?<depName>.*)\n *version: (?<currentValue>.*)"
            ],
            "datasourceTemplate": "github-releases"
        }
    ]
}

The regex manager is for renovating pinned nextcloud app versions. We want to include that in this repo as a "library config" (not included by default), so we can share it among the nextcloud and stackspin repos.