Document environment variables

**Describe the problem you have/What new integration you would like**

The official documentation barely mentions any info on environment variables used by esphome. I went through the code and collected them, it would be nice if someone, preferable the one who added it, could write a short description for each.

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

**Additional context**

### ESPHOME_BUILD_PATH

https://esphome.io/components/esphome.html

build_path (Optional, string): Customize where ESPHome will store the build files for your node. By default, ESPHome puts the PlatformIO project it uses to build the firmware in the .esphome/build/ (or into path from ESPHOME_BUILD_PATH environment variable if specified) directory, but you can customize this behavior using this option. Official docker image automatically use /build folder as default one in case it is mounted to it.

### ESPHOME_TRUSTED_DOMAINS

https://github.com/esphome/esphome/pull/6845

A comma separated list of allowed domains.

If the ESPHOME_TRUSTED_DOMAINS is set, the overriden check_origin compares it with the http origin header, otherwise it calls the parent and works just like before.

### USERNAME, PASSWORD

UI authentication

### ESPHOME_DASHBOARD_RELATIVE_URL
### ESPHOME_DASHBOARD_USE_PING
### ESPHOME_STREAMER_MODE
### ESPHOME_DASHBOARD_USE_MQTT
### DISABLE_HA_AUTHENTICATION

(dashboard/settings.py, happened during dashboard.py split, cannot find origin)

https://github.com/esphome/esphome/pull/461

???

### ESPHOME_IS_HA_ADDON

(is_ha_addon: helpers.py, core/__init__.py)

???

### ESPHOME_DASHBOARD_MQTT_USERNAME
### ESPHOME_DASHBOARD_MQTT_PASSWORD
### ESPHOME_DASHBOARD_MQTT_BROKER
### ESPHOME_DASHBOARD_MQTT_PORT

(mqtt.py)

???

### SUPERVISOR_TOKEN

(dashboard/web_server.py)

???

### ESPHOME_QUICKWIZARD (ENV_NOGITIGNORE)
### ESPHOME_NOGITIGNORE (ENV_DEV)

(wizard.py)
(dashboard/web_server.py)

???

### ESPHOME_DATA_DIR

(core/__init__.py)
(is_ha_addon ? /data : ESPHOME_DATA_DIR ? ESPHOME_DATA_DIR : .esphome)

???

Read the existing ESPHome component documentation first, then inspect the referenced locations: dashboard/settings.py, helpers.py, core/__init__.py, mqtt.py, dashboard/web_server.py, and wizard.py. Document each listed environment variable with its purpose and usage, covering the variables whose descriptions are currently marked as unknown.

Written by the indexing model from the issue text.