Guides still import on, fn, etc

[tcjr]

Ember 7.1 now includes several built-in helpers/modifiers that were previously imported. The docs should be updated to reflect the new template behavior.

TODO:

• Identify the code blocks (gjs/gts) where helpers/modifiers are imported and remove the imports
• Update the section on importing built-ins
• Update the section on keywords
• Possibly add some prose describing that behavior changed as of 7.1
• Update the section on helper functions, particularly the wording on the CTA.