Various Mermaid improvements (#18776) (#18780)

* Various Mermaid improvments

- Render into iframe for improved security
- Use built-in dark theme instead of color inversion
- Remove flexbox attributes, resulting in more consistent size rendering
- Update API usage and update to latest version

* restart ci

* misc tweaks

* remove unneccesary declaration

* make it work without allow-same-origin, add loading=lazy

* remove loading attribute, does not seem to work

* rename variable

* skip roundtrip to DOM for rendering

* don't guess chart height

* update comment to make it clear it's intentional

* tweak

* replace deprecated 'scrolling' property

* remove unused css file

Co-authored-by: Lunny Xiao <xiaolunwen@gmail.com>

Co-authored-by: Lunny Xiao <xiaolunwen@gmail.com>

web_src/less/markup/content.less

@@ -536,6 +536,14 @@
   }
 }
 
+.markup-render {
+  display: block;
+  border: none;
+  width: 100%;
+  height: var(--height-loading); // actual height is set in JS after loading
+  overflow: hidden;
+}
+
 .markup-block-error {
   margin-bottom: 0 !important;
   border-bottom-left-radius: 0 !important;