Round to significant digits filter

[gapple]

Description:

New filter to round to significant digits, useful for sensors with a dynamic range of multiple orders of magnitude. e.g.

Original Value	round_to_significant_digits: 2	round_to_significant_digits: 4
123456.789	120000	123500
123.4567	120	123.5
12.3456	12	12.35
1.23456	1.2	1.235
0.123456	0.12	0.1235

Related issue (if applicable): fixes

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

• esphome/esphome#11157

Checklist:

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

• [ ] I am merging into current because this is a fix, change and/or adjustment in the current documentation and is not for a new component or feature.

• [ ] Link added in /components/index.rst when creating new documents for new components or cookbook.

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 UPPER_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 images/ folder of this repository.

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

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

@esphomebot generate image DHT22

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