tpl/images: Add images.QR function

[jmooring]

Closes #13205

Note that this uses (create.Client).FromString under the covers, which seemed a lot simpler than creating a FromBytes or FromImage method.

Site and theme authors should test the rendered image per the documentation:

Always test the rendered QR code after resizing, both on-screen and in print.

[jmooring]

I'll handle styling of the example images on the docs site, so if you build the docs from this repo the examples will be stretched.

[mrusme]

Just in case anyone is stumbling upon this feature -- thank you for it btw! -- and is thinking "dammit, no way to change the colors of the QR code to adjust for dark/light themes!", I got you covered:

      <div id="qrcode">
      {{ with (images.QR .Permalink (dict "level" "high" "scale" 3)) }}
        <img src="{{ .RelPermalink }}"
            width="{{ .Width }}"
            height="{{ .Height }}"
            alt="QR code linking to {{ $.Permalink }}"
            class="qrcode-light"
            loading="lazy">
        {{ with . | images.Filter images.Invert }}
          <img src="{{ .RelPermalink }}"
              width="{{ .Width }}"
              height="{{ .Height }}"
              alt="QR code linking to {{ $.Permalink }}"
              class="qrcode-dark"
              loading="lazy">
        {{ end }}
      {{ end }}
      </div>

@media (prefers-color-scheme: dark) {
  #qrcode > .qrcode-light {
    display: none;
  }
}

@media (prefers-color-scheme: light) {
  #qrcode > .qrcode-dark {
    display: none;
  }
}