docs: Make explicit that Lambda Layers work independently of frameworks

[heitorlessa]

What were you initially searching for in the docs?

A customer wasn't sure if Lambda Layers could be used without the documented frameworks.

Is this related to an existing part of the documentation? Please share a link

Homepage->Layers->Public ARNs (#public-arns)

Describe how we could make it clearer

Add an explicit mention about Lambda Layers being pre-packages dependencies, and a link to Lambda Layers documentation on how to add via the Console

If you have a proposed update, please share it here

[heitorlessa]

Now available in the docs.