Add RTL rendering support to Markdown (#24816)

Support RTL content in Markdown:


![image](https://github.com/go-gitea/gitea/assets/115237/dedb1b0c-2f05-40dc-931a-0d9dc81f7c97)

Example document:
https://try.gitea.io/silverwind/symlink-test/src/branch/master/bidi-text.md
Same on GitHub:
https://github.com/silverwind/symlink-test/blob/master/bidi-text.md

`dir=auto` enables a browser heuristic that sets the text direction
automatically. It is the only way to get automatic text direction.

Ref: https://codeberg.org/Codeberg/Community/issues/1021

---------

Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>

web_src/css/base.css

@@ -1091,6 +1091,7 @@ a.label,
   color: var(--color-text);
   background: var(--color-box-body);
   border-color: var(--color-secondary);
+  text-align: start; /* Override fomantic's `text-align: left` to make RTL work via HTML `dir="auto"` */
 }
 
 .ui.table th,

web_src/css/markup/content.css

@@ -23,9 +23,9 @@
 }
 
 .markup .anchor {
+  float: left;
   padding-right: 4px;
   margin-left: -20px;
-  line-height: 1;
   color: inherit;
 }
 
@@ -37,6 +37,10 @@
   outline: none;
 }
 
+.markup h1 .anchor {
+  margin-top: -2px; /* re-align to center */
+}
+
 .markup h1 .anchor .svg,
 .markup h2 .anchor .svg,
 .markup h3 .anchor .svg,