Document proposed alt_text properties. (#24990)

* alt_text: picture, picture-entity, header-footer

* Revert picture-entity alt_text prop; this feature will be revisited.

* fix mistaken removal of endconfiguration directive

* Improve wording of header-footer alt text property docs.

Co-authored-by: Steve Repsher <steverep@users.noreply.github.com>

* Improve wording of picture alt text property docs.

Co-authored-by: Steve Repsher <steverep@users.noreply.github.com>

* Add link to resource

* Move up and add link to resource

Co-authored-by: Steve Repsher <steverep@users.noreply.github.com>

source/_dashboards/picture.markdown

@@ -21,6 +21,10 @@ image:
   required: true
   description: "The URL of an image. When you want to store images in your Home Assistant installation use the [hosting files documentation](/integrations/http/#hosting-files). After storing your files, use the `/local` path, for example, `/local/filename.jpg`."
   type: string
+alt_text:
+  required: false
+  description: Alternative text for the image. This is necessary for users of assistive technology. The [W3C images tutorial](https://www.w3.org/WAI/tutorials/images/) provides simple guidance for writing alternative text.
+  type: string
 theme:
   required: false
   description: Override the used theme for this card with any loaded theme. For more information about themes, see the [frontend documentation](/integrations/frontend/).

source/dashboards/header-footer.markdown

@@ -28,6 +28,10 @@ image:
   required: true
   description: The URL of an image.
   type: string
+alt_text:
+  required: false
+  description: Alternative text for the image. This is necessary for users of assistive technology. The [W3C images tutorial](https://www.w3.org/WAI/tutorials/images/) provides simple guidance for writing alternative text.
+  type: string  
 tap_action:
   required: false
   description: Action taken on card tap. See [action documentation](/dashboards/actions/#tap-action).