[📑 Docs]: Improve documentation related to `File templates` feature

[derberg]

What Dev Docs changes are you proposing?

• we need to make it clear that https://github.com/asyncapi/generator/blob/master/docs/file-templates.md is a feature for Nunjucks render engine only
• we need a new document, or refactor the one about File templates and explain how to do the same in react

we have at least comparizon from the perspective of "template for templates". So we see file templates in use in https://github.com/asyncapi/template-for-generator-templates/tree/nunjucks/template/schemas in times of nunjucks and then after conversion to react https://github.com/asyncapi/template-for-generator-templates/blob/master/template/schemas/schema.js

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