text: Title: clarify in the docs that keys in the product field can be anything
  Body: I got a user who asked me if their `pipeline.yaml` looked well and I noticed they had something like this:
  
  ```yaml
  tasks:
     # ... a bunch of tasks
     - source: fit.py
        product:
          nb: path/to/report.html
          data: path/to/data.csv
          data1: path/to/data.csv
          data2: path/to/data.csv
  ```
  
  The user was confused and thought that the naming should be name, name1, name2. So we should add a little note in the basic concepts guide to explicitly say that keys can have any name. (e.g., data, data_clean, model, etc)
  
  https://docs.ploomber.io/en/latest/get-started/basic-concepts.html
  