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

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

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

assets/warning-css.html

@@ -0,0 +1,27 @@
+<style>
+  #tracing-warning-header {
+    z-index: 400;
+    position: fixed;
+    background-color: orange;
+    left: 0;
+    top: 0;
+    right: 0;
+    height: 50px;
+    display: block;
+    color: black;
+    font-size-adjust: 0.55;
+    line-height: 0.6;
+    padding-top: 10px;
+    padding-left: 230px;
+  }
+
+  body {
+    /* add top padding to fit the warning header */
+    padding-top: 50px !important;
+  }
+
+  .sidebar {
+    /* add top padding to fit the warning header */
+    margin-top: 50px !important;
+  }
+</style>

netlify.toml

@@ -11,6 +11,9 @@
     --force-warn renamed-and-removed-lints \
     --cfg docsrs \
     --cfg tracing_unstable
+    --html-before-content /opt/build/repo/assets/warning.html \
+    --html-in-header /opt/build/repo/assets/noindex.html \
+    --html-in-header /opt/build/repo/assets/warning-css.html \
     """
   RUSTFLAGS="--cfg tracing_unstable"