doc: css: add auto overflow on admonitions

Setting overflow:auto adds a scrollbar to admonitions if the content
overflows. Some adminitions overflow specially on mobile devices.

Signed-off-by: Gerard Marull-Paretas <gerard.marull@nordicsemi.no>
diff --git a/doc/_static/css/custom.css b/doc/_static/css/custom.css
index b3368b5..b8389d2 100644
--- a/doc/_static/css/custom.css
+++ b/doc/_static/css/custom.css
@@ -535,6 +535,7 @@
 .rst-content .admonition.seealso {
     background-color: var(--admonition-note-background-color);
     color: var(--admonition-note-color);
+    overflow: auto;
 }
 
 .rst-content .admonition.note .admonition-title,
@@ -548,6 +549,7 @@
 .rst-content .admonition.warning {
     background-color: var(--admonition-attention-background-color);
     color: var(--admonition-attention-color);
+    overflow: auto;
 }
 
 .rst-content .admonition.attention .admonition-title,
@@ -560,6 +562,7 @@
 .rst-content .admonition.danger {
     background-color: var(--admonition-danger-background-color);
     color: var(--admonition-danger-color);
+    overflow: auto;
 }
 
 .rst-content .admonition.danger .admonition-title {
@@ -571,6 +574,7 @@
 .rst-content .admonition.important {
     background-color: var(--admonition-tip-background-color);
     color: var(--admonition-tip-color);
+    overflow: auto;
 }
 
 .rst-content .admonition.tip .admonition-title,