docs: fix some of the Sphinx warnings

docs/api/ledc.rst

@@ -22,15 +22,6 @@ Header Files
 
   * `driver/include/driver/ledc.h <https://github.com/espressif/esp-idf/blob/master/components/driver/include/driver/ledc.h>`_
 
-Data Structures
-^^^^^^^^^^^^^^^
-
-.. doxygenstruct:: ledc_channel_config_t
-   :members:
-
-.. doxygenstruct:: ledc_timer_config_t
-   :members:
-
 Macros
 ^^^^^^
 

docs/api/sigmadelta.rst

@@ -1,5 +1,5 @@
 Sigma-delta Modulation
-========
+======================
 
 Overview
 --------