docs: admin-guide: add bcache documentation

The bcache.txt is already in ReST format. So, move it to the
admin guide, where it belongs.

Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

Documentation/00-INDEX

@@ -64,8 +64,6 @@ auxdisplay/
 	- misc. LCD driver documentation (cfag12864b, ks0108).
 backlight/
 	- directory with info on controlling backlights in flat panel displays
-bcache.txt
-	- Block-layer cache on fast SSDs to improve slow (raid) I/O performance.
 block/
 	- info on the Block I/O (BIO) layer.
 blockdev/

Documentation/admin-guide/index.rst

@@ -60,6 +60,7 @@ configure specific aspects of kernel behavior to your liking.
    mono
    java
    ras
+   bcache
    pm/index
    thunderbolt
    LSM/index