docs: Add copywriting and design tweaks (#17514)

Follow up to https://github.com/zed-industries/zed/pull/17505. This one
contains a bit more copywriting adjustments. Figured we were using the
"You can do x..." sentence shape quite frequently, so tried to kickstart
reducing that slightly. There are also more images not loading in
complement to the one I removed, but I'm not fully sure why that's the
case.

--- 

Release Notes:

- N/A

docs/theme/css/general.css

@@ -235,7 +235,7 @@ blockquote {
   margin: auto;
   margin-top: 1rem;
   padding: 1rem 1.25rem;
-  color: var(--fg);
+  color: #000;
   background-color: var(--quote-bg);
   border: 1px solid var(--quote-border);
 }
@@ -268,6 +268,7 @@ blockquote .warning:before {
 .warning {
   margin: auto;
   padding: 1rem 1.25rem;
+  color: #000;
   background-color: var(--warning-bg);
   border: 1px solid var(--warning-border);
 }
@@ -311,8 +312,10 @@ kbd {
   margin-block-start: 2em;
 }
 .footnote-definition {
-  font-size: 0.9em;
+  font-size: 1.4rem;
   margin: 0.5em 0;
+  border-bottom: 1px solid;
+  border-color: var(--border-light);
 }
 .footnote-definition p {
   display: inline;