Document all supported YAML extensions

**Describe the problem you have/What new integration you would like**
I miss proper docs on what features I can use in YAML files on top of the specification (anchors, overrides, includes, …). This docs should include examples for common use cases.

**Please describe your use case for this integration and alternatives you've tried:**

I would like to have common settings in shared config files, so each device's file has only it's specific settings plus customizations and overrides.

**Additional context**

I've found this chunks what could be base for proper docs:
- https://github.com/esphome/esphome/pull/888
- https://next.esphome.io/components/switch/gpio.html?highlight=yaml%20anchor
- https://next.esphome.io/guides/configuration-types.html#config-substitutions
- https://github.com/esphome/issues/issues/886

Start by reviewing the linked pull request, YAML anchor example, configuration-types page, and related feature request to inventory the supported YAML extensions and existing explanations. Done means the documentation covers each supported extension with clear common-use-case examples for shared settings, customizations, and overrides.

Written by the indexing model from the issue text.