Fix view of readme file in the home code page. (#30564)

Gitea attempts to display image file, pdf file, etc. named readme in the
home code page (but it cannot).
I think only the markdown and plain-text file should be displayed, which
is also the behavior of GitHub.

Co-authored-by: jxshin <zhujiaxinabc@gmail.com>

templates/repo/view_list.tmpl

@@ -68,6 +68,6 @@
 		{{end}}
 	</tbody>
 </table>
-{{if .ReadmeExist}}
+{{if and .ReadmeExist (or .IsMarkup .IsPlainText)}}
 	{{template "repo/view_file" .}}
 {{end}}