40C3: tours, schedule data, tour map and calendar entries

- tour-map shortcode: Leaflet map of the tour locations with routes from the
  Congress, from the map/tours sections of data/schedule/<event>.yaml; Leaflet
  and the OSMIC, Maki and Temaki icon sets vendored; 39C3 uMap data converted.
- calendar shortcode: per tour date, a pill with an .ics download (iCalendar)
  and a Google Calendar link; Font Awesome Free vendored, fa-icon partial.
- README sections for the calendar and tour map shortcodes and the map data.

assets/css/_custom.css

@@ -1,4 +1,5 @@
 @import './_schedule.css';
+@import './_tour-map.css';
 @import './_gallery.css';
 
 .header__intro_headline {
@@ -106,3 +107,34 @@ span.content-image {
   font-size: var(--fs-xs);
   color: var(--color-text-meta);
 }
+
+/* Calendar links after a tour date, see layouts/_shortcodes/calendar.html.
+   The links are at least 24 pixels high, so they are easy to hit (WCAG 2.5.8). */
+.tour-calendar {
+  display: inline-flex;
+  align-items: center;
+  gap: 0.4em;
+  margin-inline-start: 0.4em;
+  padding: 0 0.6em;
+  border: 1px solid color-mix(in oklch, var(--color-text) 25%, transparent);
+  border-radius: 999px;
+  font-family: var(--ff-alt);
+  font-size: var(--fs-xs);
+  vertical-align: middle;
+  white-space: nowrap;
+
+  a {
+    display: inline-flex;
+    align-items: center;
+    min-height: 24px;
+  }
+}
+
+.tour-calendar__separator {
+  color: var(--color-text-meta);
+}
+
+.fa-icon {
+  width: 1em;
+  height: 1em;
+}