Delete devsite docs on fields that were moved out of core into samples

[BeksOmega]

Location

https://developers.google.com/blockly/guides/create-custom-blocks/fields/built-in-fields/overview

Type

• Text
• Image or Gif
• Other

Suggested content

None content! How to use these fields will now be included in the respective packages' readmes.

Additional context

Part of #7226

[maribethb]

When we delete the content, let's either add a redirect automatically to the readme, or add a link on the page to point to the readme.