docs(fix): New external link icon workaround for mkdocs-material 9.5.5 (#3823)

This is the easiest to maintain workaround now available. Upstream continues to reject the value such a feature for accessibility.

docs/content/assets/css/customizations.css

@@ -16,12 +16,16 @@ If you want to append instead, switch `::before` to `::after`.
   src: url('../fonts/external-link.woff') format('woff');
 }
 
-/* Matches the two nav link classes that start with `http` `href` values, regular docs pages use relative URLs instead. */
-.md-tabs__link[href^="http"]::before, .md-nav__link[href^="http"]::before {
+/*
+  Since mkdocs-material 9.5.5 broke support in our docs from DMS v13.3.1, we now use our own class name,
+  which has been included for the two external nav links in mkdocs.yml via workaround (insert HTML).
+*/
+.icon-external-link::before {
   display: inline-block; /* treat similar to text */
   font-family: 'external-link';
   content:'\0041'; /* represents "A" which our font renders as an icon instead of the "A" glyph */
   font-size: 80%; /* icon is a little too big by default, scale it down */
+  margin-right: 4px;
 }
 
 /* ============================================================================================================= */

docs/mkdocs.yml

@@ -173,5 +173,5 @@ nav:
     - 'General Information': contributing/general.md
     - 'Tests': contributing/tests.md
     - 'Issues and Pull Requests': contributing/issues-and-pull-requests.md
-  - 'DockerHub': https://hub.docker.com/r/mailserver/docker-mailserver/
-  - 'GHCR': https://github.com/docker-mailserver/docker-mailserver/pkgs/container/docker-mailserver
+  - '<span class="icon-external-link"></span>DockerHub': https://hub.docker.com/r/mailserver/docker-mailserver/
+  - '<span class="icon-external-link"></span>GHCR': https://github.com/docker-mailserver/docker-mailserver/pkgs/container/docker-mailserver