[coap_client] Docs for Client New Component

[rwrozelle]

Description:

Implements a new component coap_client

The CoAP Client component allows an ESPHome device to send CoAP messages to remote CoAP servers over UDP or TCP.
This is useful for integrating ESPHome devices with:

• Constrained IoT devices/sensors
• CoAP-enabled home automation systems
• Local microcontrollers using lightweight UDP messaging
• Any endpoint that exposes CoAP resources (GET/POST/etc.)

Related issue (if applicable): fixes

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

• [coap_client] Client New Component Part 1 esphome#12009

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

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