src/styles: set max-width on item page fields

Set max-width of item-page-field components to 75 characters. This
is in line with some best practices regarding readability, though
may need to be adjusted in the future.

See the following recommendations:
  - 50 to 75 characters: https://baymard.com/blog/line-length-readability
  - 60 to 80 characters: https://pimpmytype.com/line-length-line-height/
  - 50 to 75 characters: https://www.uxpin.com/studio/blog/optimal-line-length-for-readability/
  - 50 to 75 characters: https://medium.com/@wblekhoa/talk-aboutthe-optimal-length-of-text-in-ux-ui-525e689f0b71
  - Less than 80 characters: https://www.w3.org/WAI/WCAG21/Understanding/visual-presentation.html

Author: alanorth

src/styles/_custom_variables.scss

@@ -165,6 +165,7 @@
 
 
   --ds-item-page-img-field-default-inline-height: 24px;
+  --ds-item-page-field-max-width: 75ch;
 
   --ds-qa-logo-width: 100px;
 

src/styles/_global-styles.scss

@@ -573,3 +573,7 @@ ngb-tooltip-window {
   mask-size: contain;
   background-color: currentColor;
 }
+
+.item-page-field {
+  max-width: var(--ds-item-page-field-max-width);
+}