docs: (custom.css): add caption feature and...

make immediate use of it in `fs.rst`.  This gives a "hint" for the
reader on how the image may be managed to see it better and
zoom in on its parts and move around after being zoomed in.

Also added a long-overdue extra space above headings of
various sizes.  <h1> is not in that list because <h1> is always
at the top of the page, and so should not have extra space
above it.

docs/src/details/main-modules/fs.rst

@@ -366,6 +366,11 @@ so the driver needs to implement more callbacks when the cache is enabled.
              --> P["copy the required bytes
                     to the destination buffer"]
 
+.. container:: lvgl-figure-caption
+
+    (Use mouse wheel to zoom in, and drag image to pan it.)
+
+
 ``lv_fs_write`` :sub:`(behavior when cache is enabled)`
 -------------------------------------------------------