driver: sigma-delta

1. add sigma-delta code and example.
2. add gpio output signal in io matrix.

Squashed commits:
[a25e7d4] minor changes for doc
[6a03a1e] minor changes on comments and doc.
[97dd9e4] Add doc.
update index.rst
change example index

components/driver/gpio.c

@@ -292,6 +292,7 @@ esp_err_t gpio_config(gpio_config_t *pGPIOConfig)
             }
             if ((pGPIOConfig->mode) & GPIO_MODE_DEF_OUTPUT) {
                 output_en = 1;
+                gpio_matrix_out(io_num, SIG_GPIO_OUT_IDX, 0, 0);
                 gpio_output_enable(io_num);
             } else {
                 gpio_output_disable(io_num);