Documentation: i2c: add testunit docs to index

Fixes: a8335c64c5 ("i2c: add slave testunit driver")
Reported-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Signed-off-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
Signed-off-by: Wolfram Sang <wsa@kernel.org>

Documentation/i2c/index.rst

@@ -47,6 +47,7 @@ Slave I2C
 
    slave-interface
    slave-eeprom-backend
+   slave-testunit-backend
 
 Advanced topics
 ===============