Add RTL rendering support to Markdown (#24816)

Support RTL content in Markdown:


![image](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

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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,