Add /mirrors and /mirrors.json

Generate lists of official postmarketOS mirrors as HTML and JSON, as replacement of the manually updated HTML file at https://mirrors.postmarketos.org

• Let's redirect this to postmarketos.org/mirrors once merged.

The idea is to use this from pmbootstrap to query the current list of available mirrors (https://gitlab.com/postmarketOS/pmbootstrap/-/merge_requests/2035).

Generated files look like this:

https://postmarketos.org/mirrors.json

{
    "mirror.postmarketos.org": {
        "urls": [
            "https://mirror.postmarketos.org/",
            "rsync://mirror.postmarketos.org::postmarketos"
        ],
        "location": "Falkenstein, Germany",
        "bandwidth": "1 Gbps"
    },
    "postmarketos.craftyguy.net": {
        "urls": [
            "http://postmarketos.craftyguy.net/"
        ],
        "location": "Santa Clara, CA, USA",
        "bandwidth": "750 Mbps"
    },
    "southeastasia.cloudapp.azure.com": {
        "urls": [
            "http://postmarketos-mirror.southeastasia.cloudapp.azure.com/"
        ],
        "location": "Singapore"
    },
    "eastus2.cloudapp.azure.com": {
        "urls": [
            "http://postmarketos-mirror.eastus2.cloudapp.azure.com/"
        ],
        "location": "Virginia, USA"
    },
    "southafricanorth.cloudapp.azure.com": {
        "urls": [
            "http://postmarketos-mirror.southafricanorth.cloudapp.azure.com/"
        ],
        "location": "Johannesburg, Africa"
    },
    "mirror.math.princeton.edu": {
        "urls": [
            "http://mirror.math.princeton.edu/pub/postmarketos/",
            "https://mirror.math.princeton.edu/pub/postmarketos/",
            "rsync://mirror.math.princeton.edu/pub/postmarketos"
        ],
        "location": "Princeton, USA",
        "bandwidth": "4x10 Gbps"
    }
}

https://postmarketos.org/mirrors

CC @MartijnBraam, who created the current mirror list. I think the existing table looks a bit nicer, but it doesn't work as well on mobile because it is a table. I've focused on having something functional without spending too much time on it. Feel free to push further commits / amend the existing commit to make it prettier as you see fit.