feat(CSS): Add JetBrains Mono font import and set it as base font family

This commit adds an `@import` statement to import the 'JetBrains Mono' font from Google Fonts. It also sets the imported font as the base font family for the project by updating the `--ifm-font-family-base` variable in the `:root` selector. This change aims to improve the overall typography and readability of the website, especially for code snippets and content-centric sections.

src/css/custom.scss

@@ -4,6 +4,8 @@
  * work well for content-centric websites.
  */
 
+ @import url('https://fonts.googleapis.com/css2?family=JetBrains+Mono&display=swap');
+
 /* You can override the default Infima variables here. */
 :root {
   --ifm-color-primary: #2e8555;
@@ -15,6 +17,7 @@
   --ifm-color-primary-lightest: #3cad6e;
   --ifm-code-font-size: 95%;
   --docusaurus-highlighted-code-line-bg: rgba(0, 0, 0, 0.1);
+  --ifm-font-family-base: 'JetBrains Mono';
 }
 
 /* For readability concerns, you should choose a lighter palette in dark mode. */