Add endstop valve platform documentation

[exciton]

Description

Adds endstop valve platform docs

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

• [endstop] Add valve to endstop component (cover/valve refactor 4/5) esphome#16667

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 /src/content/docs/components/index.mdx 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 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 /public/images/ folder of this repository.

4. Use the image in your component's index table entry in /src/content/docs/components/index.mdx.

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

@esphomebot generate image dht22

Note: All images used in ImgTable components must be placed in /public/images/ as the component resolves them to absolute paths.

[github-actions]

ImgTable blocks are not in alphabetical order

Found 1 ImgTable block(s) with incorrect ordering below Network Protocols.
Each table has at most one Core item (matching the section name, e.g. "Sensor Core" for the Sensor section), pinned first, followed by Template items (names starting with "Template "), then all remaining items sorted alphabetically.

You can fix this automatically by running:

node script/check_component_index.mjs --fix

See the inline suggestions below for the correct order in each section.

[esphome]

Please take a look at the requested changes, and use the Ready for review button when you are done, thanks 👍

Learn more about our pull request process.

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Component index ordering has been fixed — dismissing review.