Add colours and border-left to "version modified" API helpers

This is inspired by a similar change made to Python's documentation
theme, which helps highlight this important information.

src/furo/assets/styles/content/_api.sass

@@ -82,13 +82,39 @@ em.property
 .sig-param
   font-style: normal
 
-.versionmodified
-  font-style: italic
-div.versionadded, div.versionchanged, div.deprecated
+div.versionadded,
+div.versionchanged,
+div.deprecated,
+div.versionremoved
+  border-left: 0.1875rem solid
+  border-radius: 0.125rem
+
+  padding-left: 0.75rem
+
   p
     margin-top: 0.125rem
     margin-bottom: 0.125rem
 
+div.versionadded
+  border-color: var(--color-api-added-border)
+  .versionmodified
+    color: var(--color-api-added)
+
+div.versionchanged
+  border-color: var(--color-api-changed-border)
+  .versionmodified
+    color: var(--color-api-changed)
+
+div.deprecated
+  border-color: var(--color-api-deprecated-border)
+  .versionmodified
+    color: var(--color-api-deprecated)
+
+div.versionremoved
+  border-color: var(--color-api-removed-border)
+  .versionmodified
+    color: var(--color-api-removed)
+
 // Align the [docs] and [source] to the right.
 .viewcode-link, .viewcode-back
   float: right

src/furo/assets/styles/variables/_colors.scss

@@ -39,6 +39,16 @@
   --color-api-pre-name: var(--color-problematic);
   --color-api-paren: var(--color-foreground-secondary);
   --color-api-keyword: var(--color-foreground-primary);
+
+  --color-api-added: #21632c;
+  --color-api-added-border: #38a84d;
+  --color-api-changed: #09635b;
+  --color-api-changed-border: #0ea498;
+  --color-api-deprecated: #605706;
+  --color-api-deprecated-border: #f0d90f;
+  --color-api-removed: #b30000;
+  --color-api-removed-border: #ff5c5c;
+
   --color-highlight-on-target: #ffffcc;
 
   // Inline code background
@@ -167,6 +177,15 @@
   --color-api-keyword: var(--color-foreground-secondary);
   --color-highlight-on-target: #333300;
 
+  --color-api-added: #3db854;
+  --color-api-added-border: #267334;
+  --color-api-changed: #b1a10b;
+  --color-api-changed-border: #6e6407;
+  --color-api-deprecated: #ff7575;
+  --color-api-deprecated-border: #c70000;
+  --color-api-removed: #ff7575;
+  --color-api-removed-border: #c70000;
+
   // Admonitions
   --color-admonition-background: #18181a;