docs: Update Sphinx; remove Markdown; add CSS

This completes the update of Sphinx and RST by removing the legacy
Markdown support. It also adds custom CSS skeleton.

Change-Id: I398a21208772cf2d2dd32dee0adac768630dd575
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/40621
Commit-Queue: Keir Mierle <keir@google.com>
Pigweed-Auto-Submit: Keir Mierle <keir@google.com>
Reviewed-by: Anthony DiGirolamo <tonymd@google.com>
Reviewed-by: Ewout van Bekkum <ewout@google.com>
diff --git a/pw_log_basic/docs.rst b/pw_log_basic/docs.rst
index 985ec50..7bdf8c5 100644
--- a/pw_log_basic/docs.rst
+++ b/pw_log_basic/docs.rst
@@ -13,7 +13,7 @@
 
 .. cpp:namespace:: pw::log_basic
 
-.. cpp:function:: void SetOutput(void (\*log_output)(std::string_view))
+.. cpp:function:: void SetOutput(void (*log_output)(std::string_view))
 
   Set the log output function, which defaults ``pw_sys_io::WriteLine``. This
   function is called with each formatted log message.