Resize the "breadcrumbs" navigation section to fit the content, and stack it below the top navigation bar (#680)

* Resize the "breadcrumbs" navigation section

* Drop spurious `nav` to reset breadcrumbs' z-index

Author: agriyakhetarpal

assets/theme-css/navbar.css

@@ -1,4 +1,3 @@
-nav,
 .navbar {
   background: var(--pst-color-background);
   position: sticky;

assets/theme-css/pst/components/_breadcrumbs.scss

@@ -1,6 +1,10 @@
 /**
  * Breadcrumbs for parent pages meant for the article header
  */
+nav[aria-label="Breadcrumb"] {
+  width: fit-content;
+}
+
 ul.bd-breadcrumbs {
   list-style: none;
   padding-left: 0;