Embed CSS entries in soong_build.html.

This will prevent it from being affected by future css location changes.

Test: m soong_docs
Fixes: 130567020
Change-Id: I80fd7b8a12a32bcea7b270423b8d8f8dc0976177
Merged-In: I80fd7b8a12a32bcea7b270423b8d8f8dc0976177
(cherry picked from commit 5f867c05)