[DOCS UPDATE] Add all supported config file formats

**Describe the bug**
Currently the [website](https://graphql-code-generator.com/docs/getting-started/codegen-config) mentions:

> To pass configuration to GraphQL Codegen, you need to simply create a codegen.yml or codegen.json file and run the codegen.

There seem to be more supported formats as stated [here](https://github.com/dotansimha/graphql-code-generator/issues/1167). I think for many people, it might be beneficial to know that it can be a `codegen.js` file too. Makes it easier when adding a token to the header for example.