docs: admin-guide/index.rst: Add pstore-blk.rst

Fix the following warning:

Documentation/admin-guide/pstore-blk.rst:
WARNING: document isn't included in any toctree

By adding 'pstore-blk.rst' to the index

Signed-off-by: Daniel W. S. Almeida <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Acked-by: Kees Cook <[e-mail]>
Signed-off-by: Jonathan Corbet <[e-mail]>

Documentation/admin-guide/index.rst

···
    init
    kdump/index
    perf/index
+   pstore-blk
 
 This is the beginning of a section with information of interest to
 application developers.  Documents covering various aspects of the kernel