ENH - Modernize font stack (#2263)

Fixes #2262

The emoji part is from this note in the README:
https://github.com/system-fonts/modern-font-stacks#emoji-support

---------

Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>

Author: flying-sheep

src/pydata_sphinx_theme/assets/styles/variables/_fonts.scss

@@ -32,13 +32,15 @@ html {
   --pst-font-weight-heading: 600;
 
   // Font family
-  // These are adapted from https://systemfontstack.com/ */
+  // https://modernfontstacks.com/?stack=system-ui
   --pst-font-family-base-system:
-    -apple-system, "BlinkMacSystemFont", "Segoe UI", "Helvetica Neue", "Arial",
-    sans-serif, "Apple Color Emoji", "Segoe UI Emoji", "Segoe UI Symbol";
+    system-ui, sans-serif, "Apple Color Emoji", "Segoe UI Emoji",
+    "Segoe UI Symbol", "Noto Color Emoji";
+
+  // https://modernfontstacks.com/?stack=monospace-code
   --pst-font-family-monospace-system:
-    "SFMono-Regular", "Menlo", "Consolas", "Monaco", "Liberation Mono",
-    "Lucida Console", monospace;
+    ui-monospace, "Cascadia Code", "Source Code Pro", "Menlo", "Consolas",
+    "DejaVu Sans Mono", monospace;
   --pst-font-family-base: var(--pst-font-family-base-system);
   --pst-font-family-heading: var(--pst-font-family-base-system);
   --pst-font-family-monospace: var(--pst-font-family-monospace-system);