[sntp] Add docs for smooth_sync config option

[rogerfachini]

Description:

Updates documentation for the SNTP component to include the smooth_sync parameter introduced in the linked PR below

Related issue (if applicable): fixes

Pull request in esphome with YAML changes (if applicable):

• [sntp] Add smooth_sync config option esphome#13630

Checklist:

• I am merging into next because this is new documentation that has a matching pull-request in esphome as linked above.

New Component Images

If you are adding a new component to ESPHome, you can automatically generate a standardized black and white component name image for the documentation.

To generate a component image:

1. Comment on this pull request with the following command, replacing component_name with your component name in lower_case format with underscores (e.g., bme280, sht3x, dallas_temp):

   @esphomebot generate image component_name
   

2. The ESPHome bot will respond with a downloadable ZIP file containing the SVG image.

3. Extract the SVG file and place it in the /static/images/ folder of this repository.

4. Use the image in your component's index table entry in /components/_index.md.

Example: For a component called "DHT22 Temperature Sensor", use:

@esphomebot generate image dht22

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