doc: Add /proc/bootconfig to proc.rst

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

Link: https://lore.kernel.org/all/20231005171747.541123-3-paulmck@kernel.org/

Reported-by: Masami Hiramatsu <mhiramat@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>