fix: Mark tel: links as safe (#189)

## Description
By default, Hugo only allows using `http:`, `https:`, and `mailto:`.
URLs using other schemes are replaced with `#ZgotmplZ`. To allow using
`tel:`, we have to declare it as safe using `| safeURL`. This marks
every URL as safe which should be fine, since all content inside the
repo is trusted.
See https://gohugo.io/functions/safe/url/ for more info.

Closes #185

<!-- Add a short description here / Eine kurze Beschreibung hier
einfügen -->

## Checklist
<!-- Check fields with: [x] / Abhaken von Punkten: [x] -->

- [x] Check the License of new pictures (non-commercial use without
attribution) <!-- Die Lizenz neuer Bilder geprüft (nicht-kommerzielle
Nutzung ohne Namensnennung) -->

The content was modified in the following languages: <!-- Der Inhalt
wurde für die folgenden Sprachen angepasst -->
- [ ] English
- [ ] German

---------

Co-authored-by: Moritz Weber <kontakt@moritz-weber.net>

Author: zusorio

layouts/_default/_markup/render-link.html

@@ -2,5 +2,5 @@
 {{- if strings.HasPrefix $url "http" -}}
 {{ partial "link" . }}
 {{- else -}}
-<a href="{{ $url }}">{{ .Text }}</a>
+<a href="{{ $url | safeURL }}">{{ .Text }}</a>
 {{- end -}}