Add pull request manually merge instruction (#13840)

* add pull request command line instructions

* Add pull request manually merge instuction

* Fix styles

* Fix lint

* Move inline style to class file

* add space between merge button and hint text

* Add sentence end charcter

* Change the language file

* adjust secondary bg

* further adjustment

Co-authored-by: silverwind <me@silverwind.io>

web_src/less/_base.less

@@ -99,7 +99,8 @@
   --color-markdown-code-block: #00000008;
   --color-button: #ffffff;
   --color-code-bg: #ffffff;
-  --color-shadow: #00000024;
+  --color-markdown-code-block: #00000010;
+  --color-secondary-bg: #f4f4f4;
 }
 
 :root:lang(ja) {
@@ -1691,6 +1692,11 @@ a.ui.basic.label:hover {
   border-color: var(--color-secondary);
 }
 
+.ui.secondary.segment {
+  background: var(--color-secondary-bg);
+  color: var(--color-text-light);
+}
+
 .ui.attached.header {
   position: relative;
   background: var(--color-box-header);