Add percpu-counter-tree to the core-api index to prevent a docs warning:

Documentation/core-api/percpu-counter-tree.rst: WARNING: document isn't included in any toctree [toc.not_included]

Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
---
Cc: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
M:	Jonathan Corbet <corbet@lwn.net>
R:	Shuah Khan <skhan@linuxfoundation.org>
L:	linux-doc@vger.kernel.org


 Documentation/core-api/index.rst |    1 +
 1 file changed, 1 insertion(+)

--- linux-next-20260313.orig/Documentation/core-api/index.rst
+++ linux-next-20260313/Documentation/core-api/index.rst
@@ -57,6 +57,7 @@ Library functionality that is used throu
    min_heap
    parser
    list
+   percpu-counter-tree
 
 Low level entry and exit
 ========================