docs: Migrating Direct Boot preview docs into DAC

For Android N release, migrating and updating Direct Boot
docs into regular DAC docs. Updated TOC and added redirect.
Removed preview/features doc.

Bug: 30256895
Change-Id: I5c6d457dc69c911199f86f0e0ed93ec7d6244779
diff --git a/docs/html/_redirects.yaml b/docs/html/_redirects.yaml
index b6d28dc..5629ddd 100644
--- a/docs/html/_redirects.yaml
+++ b/docs/html/_redirects.yaml
@@ -1212,3 +1212,5 @@
   to: /training/tv/playback/picture-in-picture.html
 - from: /preview/features/tv-recording-api.html
   to: /training/tv/tif/content-recording.html
+- from: /preview/features/direct-boot.html
+  to: /training/articles/direct-boot.html