docs: fix warning styles in html-in-header (#2449)

## Motivation

The documentation under https://tracing.rs/tracing/ is missing CSS
styles. Fixes #2444 

## Solution

Used the `--html-in-header` rustdoc flag instead of `--extend-css`.
Converted the css file into an html file.

assets/warning-css.html

@@ -1,3 +1,4 @@
+<style>
   #tracing-warning-header {
     z-index: 400;
     position: fixed;
@@ -23,3 +24,4 @@ body {
     /* add top padding to fit the warning header */
     margin-top: 50px !important;
   }
+</style>

netlify.toml

@@ -12,7 +12,7 @@
     --cfg docsrs \
     --html-before-content /opt/build/repo/assets/warning.html \
     --html-in-header /opt/build/repo/assets/noindex.html \
-    --extend-css /opt/build/repo/assets/warning.css \
+    --html-in-header /opt/build/repo/assets/warning-css.html \
     """
 
 [[redirects]]