doc: Add /proc/bootconfig to proc.rst

Add /proc/bootconfig description to Documentation/filesystems/proc.rst.

Reported-by: Masami Hiramatsu <mhiramat@kernel.org>
Link: https://lore.kernel.org/all/20230731233130.424913-3-paulmck@kernel.org/

Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
Acked-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>
Signed-off-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>