[mapping] Update docs

[clydebarrow]

Description:

Related issue (if applicable): fixes

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

• [mapping] Use custom allocator esphome#9972

Checklist:

• 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

[Copilot]

Pull Request Overview

This PR updates the documentation for the mapping component to improve readability and add new functionality details. The changes include formatting improvements, clarification of mapping usage, and examples of runtime map updates.

• Line wrapping and formatting improvements for better readability
• Added documentation for the get() function and runtime map updates
• Extended the complex example with additional entries and usage patterns

Comments suppressed due to low confidence (1)

components/mapping.rst:95

• Mismatched brackets: there's an opening parenthesis after get( but a closing square bracket ] instead of a closing parenthesis. It should be color_map.get(random_uint32() % 3).

          auto color = color_map.get(random_uint32() % 3]); # Uses get() to index the color_map

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