clarify in the docs that keys in the product field can be anything

I got a user who asked me if their pipeline.yaml looked well and I noticed they had something like this:

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

[mhered]

Cool. Me pongo a ello.

[mhered]

I made the changes and opened PR#1033