doc: css: make sidebar style available globally

The sidebar used for the "wikipedia"-style card in the documentation
pages of board is actually something that is worth reusing elsewhere
so move the corresponding styles from board.css to global custom.css

Signed-off-by: Benjamin Cabé <benjamin@zephyrproject.org>
diff --git a/doc/_extensions/zephyr/domain/static/css/board.css b/doc/_extensions/zephyr/domain/static/css/board.css
index d2d90b9..96ee894 100644
--- a/doc/_extensions/zephyr/domain/static/css/board.css
+++ b/doc/_extensions/zephyr/domain/static/css/board.css
@@ -4,69 +4,6 @@
  */
 
 /* Board overview "card" on board documentation pages */
-.sidebar.board-overview {
-    border-radius: 12px;
-    padding: 0px;
-    background: var(--admonition-note-background-color);
-    color: var(--admonition-note-title-color);
-    border-color: var(--admonition-note-title-background-color);
-}
-
-@media screen and (max-width: 480px) {
-    .sidebar.board-overview {
-        float: none;
-        margin-left: 0;
-    }
-}
-
-.sidebar.board-overview {
-    color: var(--admonition-note-color);
-}
-
-.sidebar.board-overview .sidebar-title {
-    font-family: var(--header-font-family);
-    background: var(--admonition-note-title-background-color);
-    color: var(--admonition-note-title-color);
-    border-radius: 12px 12px 0px 0px;
-    margin: 0px;
-    text-align: center;
-}
-
-.sidebar.board-overview figure {
-    padding: 1rem;
-    margin-bottom: -1rem;
-}
-
-.sidebar.board-overview figure img {
-    height: auto !important;
-}
-
-.sidebar.board-overview figure figcaption p {
-    margin-bottom: 0px;
-}
-
-.sidebar.board-overview dl.field-list {
-    align-items: center;
-    margin-top: 12px !important;
-    margin-bottom: 12px !important;
-    grid-template-columns: auto 1fr !important;
-    padding-right: 1em;
-}
-
-.sidebar.board-overview dl.field-list>dt {
-    background: transparent !important;
-}
-
-.sidebar.board-overview dl.field-list>dd {
-    margin-left: 1em;
-    text-overflow: ellipsis;
-    overflow: hidden;
-}
-
-.sidebar.board-overview dl.field-list>dd code {
-    font-size: 0.9em;
-}
-
 .sidebar.board-overview #board-github-link {
     text-align: center;
     margin-bottom: 1em;
diff --git a/doc/_static/css/custom.css b/doc/_static/css/custom.css
index 2d36541..e6c0853 100644
--- a/doc/_static/css/custom.css
+++ b/doc/_static/css/custom.css
@@ -927,6 +927,70 @@
     filter: var(--graphviz-filter);
 }
 
+/* Sidebar */
+.rst-content .sidebar {
+    border-radius: 12px;
+    padding: 0px;
+    background: var(--admonition-note-background-color);
+    color: var(--admonition-note-title-color);
+    border-color: var(--admonition-note-title-background-color);
+}
+
+@media screen and (max-width: 480px) {
+    .rst-content .sidebar {
+        float: none;
+        margin-left: 0;
+    }
+}
+
+.rst-content .sidebar {
+    color: var(--admonition-note-color);
+}
+
+.rst-content .sidebar .sidebar-title {
+    font-family: var(--header-font-family);
+    background: var(--admonition-note-title-background-color);
+    color: var(--admonition-note-title-color);
+    border-radius: 12px 12px 0px 0px;
+    margin: 0px;
+    text-align: center;
+}
+
+.rst-content .sidebar figure {
+    padding: 1rem;
+    margin-bottom: -1rem;
+}
+
+.rst-content .sidebar figure img {
+    height: auto !important;
+}
+
+.rst-content .sidebar figure figcaption p {
+    margin-bottom: 0px;
+}
+
+.rst-content .sidebar dl.field-list {
+    align-items: center;
+    margin-top: 12px !important;
+    margin-bottom: 12px !important;
+    grid-template-columns: auto 1fr !important;
+    padding-right: 1em;
+}
+
+.rst-content .sidebar dl.field-list>dt {
+    background: transparent !important;
+}
+
+.rst-content .sidebar dl.field-list>dd {
+    margin-left: 1em;
+    text-overflow: ellipsis;
+    overflow: hidden;
+}
+
+.rst-content .sidebar dl.field-list>dd code {
+    font-size: 0.9em;
+}
+
 /* Home page grid display */
 .grid {
     list-style-type: none !important;