Don't base_url-prefix external CSS/JS URLs in theme

extraCss/extraJavaScript entries that are absolute URLs (http(s):// or
protocol-relative //) were being rewritten as {base_url}{href}, producing
broken links like ../../../https://cdn.example.com/x.css.

Skip the base_url/asset rewrite for absolute external URLs in head.html and
scripts.html, emitting them verbatim. Local asset paths are unchanged.

Found while converting a real MkDocs site that references CDN-hosted CSS/JS.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: Netdocs

src/Netdocs.Theme.Material/templates/partials/head.html

@@ -33,8 +33,12 @@
     <link rel="stylesheet" href="https://fonts.googleapis.com/css?family={{ string.replace font_text ' ' '+' }}:300,300i,400,400i,700,700i%7C{{ string.replace font_code ' ' '+' }}:400,400i,700,700i&display=fallback">
     <style>:root{--md-text-font:"{{ font_text }}";--md-code-font:"{{ font_code }}"}</style>
     {{~ for href in stylesheets ~}}
+    {{~ if (string.starts_with href "http") || (string.starts_with href "//") ~}}
+    <link rel="stylesheet" href="{{ href }}">
+    {{~ else ~}}
     <link rel="stylesheet" href="{{ base_url }}{{ asset href }}">
     {{~ end ~}}
+    {{~ end ~}}
     {{~ # Helper functions for inline scripts (palette persistence, search index caching,
        # instant navigation). The vendored Material bundle references these globals during
        # init; without them it throws `__md_get is not defined`, which aborts the whole

src/Netdocs.Theme.Material/templates/partials/scripts.html

@@ -30,8 +30,12 @@
     </script>
     <script src="{{ base_url }}assets/vendor/material/javascripts/bundle.d7400e89.min.js"></script>
     {{~ for src in scripts ~}}
+    {{~ if (string.starts_with src "http") || (string.starts_with src "//") ~}}
+    <script src="{{ src }}" defer></script>
+    {{~ else ~}}
     <script src="{{ base_url }}{{ asset src }}" defer></script>
     {{~ end ~}}
+    {{~ end ~}}
     {{~ for js in inline_scripts ~}}
     <script>{{ js }}</script>
     {{~ end ~}}