Add docs for LTR390 (#979)

* Add docs

* Fix code block indentation

* Add emtpy lines for math directive

* Fix table indentation

* Fix table indentation

* Fix math indentation

* Add adafruit link

* Changed docs order to place important feature first

* Update components/sensor/ltr390.rst

Co-authored-by: Otto Winter <otto@otto-winter.com>

* Update components/sensor/ltr390.rst

Co-authored-by: Otto Winter <otto@otto-winter.com>

* Update components/sensor/ltr390.rst

Co-authored-by: Otto Winter <otto@otto-winter.com>

Co-authored-by: Otto Winter <otto@otto-winter.com>

components/sensor/ltr390.rst

@@ -0,0 +1,117 @@
+LTR390 UV and Ambient Light Sensor
+==================================================
+
+.. seo::
+    :description: Instructions for setting up LTR390 UV and light sensor
+    :image: images/ltr390-full.jpg
+
+The ``ltr390`` sensor platform  allows you to use your LTR390 UV and ambient
+light sensor
+(`datasheet <https://optoelectronics.liteon.com/upload/download/DS86-2015-0004/LTR-390UV_Final_%20DS_V1%201.pdf>`__, `Adafruit`_) with ESPHome.
+
+The :ref:`I²C Bus <i2c>` is required to be set up in your configuration for this sensor to work.
+
+.. figure:: images/ltr390-full.jpg
+    :align: center
+    :width: 80.0%
+
+.. _Adafruit: https://www.adafruit.com/product/4831
+
+.. code-block:: yaml
+
+    sensor:
+      - platform: ltr390
+        uvi:
+          name: "UV Index"
+        light:
+          name: "Light"
+
+Configuration variables:
+------------------------
+
+- **uv_index** (**Optional**): UV index (UVI). All options from :ref:`Sensor <config-sensor>`.
+- **uv** (**Optional**): Sensor counts for the UV sensor (#). All options from :ref:`Sensor <config-sensor>`.
+- **light** (**Optional**): Lux of ambient light (lx). All options from :ref:`Sensor <config-sensor>`.
+- **ambient_light** (**Optional**): Sensor counts for the Ambient light sensor (#). All options from :ref:`Sensor <config-sensor>`.
+- **gain** (**Optional**, string): Adjusts the sensitivity of the sensor. A larger value means higher sensitivity. See table below for details. Default is ``"X3"``.
+- **resolution** (**Optional**, int): ADC resolution. Higher resolutions require longer sensor integration times. See table below for details. Default is ``18``.
+- **window_correction_factor** (**Optional**, float): Window correction factor. Use larger values when using under tinted windows. Default is ``1.0``, must be ``>= 1.0``.
+- **address** (**Optional**, int): Manually specify the I²C address of the sensor. Default is `0x53`.
+- **update_interval** (**Optional**, :ref:`config-time`): The interval to check the
+  sensor. Defaults to ``60s``. It is recommended that the update interval is at least 1 second since updates can take up to 800ms when using a high resolution value.
+
+Lux and UVI Formulas
+--------------------
+
+.. math::
+
+    \text{lux} = \frac{0.6 \times \text{als}}{\text{gain} \times \text{int}} \times \text{wfac}
+
+.. math::
+
+    \text{UVI} = \frac{\text{uv}}{\text{sensitivity}} \times \text{wfac}
+
+where:
+
+- ``als`` and ``uv`` are the sensor values
+- ``gain`` is the amount of gain, see the table below for details
+- ``int`` is the integration time in 100s of ms and is tied to the resolution, see the table below for details
+- ``sensitivity`` has the value ``2300`` and is the sensor's count per UVI
+- ``wfac`` is the window correction factor
+
+Gain
+----
+
+.. list-table::
+    :widths: 25 25
+    :header-rows: 1
+
+    * - Gain Parameter
+      - gain
+    * - X1
+      - 1
+    * - X3
+      - 3
+    * - X6
+      - 6
+    * - X9
+      - 9
+    * - X18
+      - 18
+
+
+Resolution
+----------
+
+.. list-table:: Resolution
+    :widths: 25 25 10
+    :header-rows: 1
+
+    * - Resolution Parameter (bits)
+      - Integration Time (ms)
+      - int
+    * - 16
+      - 25
+      - 0.25
+    * - 17
+      - 50
+      - 0.5
+    * - 18
+      - 100
+      - 1
+    * - 19
+      - 200
+      - 2
+    * - 20
+      - 400
+      - 4
+
+See Also
+--------
+
+- :doc:`/components/sensor/bh1750`
+- :doc:`/components/sensor/tsl2561`
+- :doc:`/cookbook/temt6000`
+- :ref:`sensor-filters`
+- :apiref:`ltr390/ltr390.h`
+- :ghedit:`Edit`