Added template local development docs

[mishushakov]

• Added docs on how to develop templates locally using Docker

[dobrac]

let's describe two use cases of local development we have now:

• start with proper E2B testing/dev workflow, describing how to work with that
• then keep what you have there now, just to add another interesting use case for the build system capabilities

this way, we would

• help you with how do we envision working with the build system
• promote E2B and how easy it's working with it
• still showing you alternative ways that might be really useful