Added Certificate Installation steps for Arch Linux (#7422)

* Update concepts-certificates.md

* Update index.html

docs/src/content/concepts-certificates.md

@@ -61,6 +61,7 @@ documentation for some common platforms. The mitmproxy CA cert is located in
   `sudo security add-trusted-cert -d -p ssl -p basic -k /Library/Keychains/System.keychain ~/.mitmproxy/mitmproxy-ca-cert.pem`
 - [Ubuntu/Debian]( https://askubuntu.com/questions/73287/how-do-i-install-a-root-certificate/94861#94861)
 - [Fedora](https://docs.fedoraproject.org/en-US/quick-docs/using-shared-system-certificates/#proc_adding-new-certificates)
+- [Arch Linux](https://wiki.archlinux.org/title/Transport_Layer_Security#Add_a_certificate_to_a_trust_store)
 - [Mozilla Firefox](https://wiki.mozilla.org/MozillaRootCertificate#Mozilla_Firefox)
 - [Chrome on Linux](https://stackoverflow.com/a/15076602/198996)
 - [iOS](http://jasdev.me/intercepting-ios-traffic)  

mitmproxy/addons/onboardingapp/templates/index.html

@@ -55,6 +55,10 @@
         <li><code>mv mitmproxy-ca-cert.pem /etc/pki/ca-trust/source/anchors/</code></li>
         <li><code>sudo update-ca-trust</code></li>
     </ol>
+    <h5>Arch Linux</h5>
+    <ol>
+        <code>sudo trust anchor --store mitmproxy-ca-cert.pem</code>
+    </ol>
     {% endcall %}
     {% call entry('macOS', 'apple') %}
     <h5>Manual Installation</h5>