Add --font-weight-bold and set previous bold to 601 (#24307)

Fix #24305

According to MDN, "bold" starts from 700, some fonts do not provide
"bolding" for weight 600

https://developer.mozilla.org/en-US/docs/Web/CSS/font-weight

---------

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

web_src/css/markup/content.css

@@ -70,7 +70,7 @@
 .markup h6 {
   margin-top: 24px;
   margin-bottom: 16px;
-  font-weight: 600;
+  font-weight: var(--font-weight-bold);
   line-height: 1.25;
 }
 
@@ -248,7 +248,7 @@
   margin-top: 16px;
   font-size: 1em;
   font-style: italic;
-  font-weight: 600;
+  font-weight: var(--font-weight-bold);
 }
 
 .markup dl dd {
@@ -280,7 +280,7 @@
 }
 
 .markup table th {
-  font-weight: 600;
+  font-weight: var(--font-weight-bold);
 }
 
 .markup table th,