better document how to guess a product type

[sbrunato]

see https://github.com/CS-SI/eodag/issues/242#issuecomment-877068032

• [ ] Add more examples showing how to guess the product type in https://eodag.readthedocs.io/en/latest/notebooks/api_user_guide/4_search.html#Guess-a-product-type
• [ ] Link the guess_product_type() method in the API reference documentation
• [ ] Document whoosh query language available through guess_product_type() method (operators, wildcard)
• [ ] Show an example on how to guess the product type while performing a CLI search, for example:

eodag search --platform SENTINEL2 --processingLevel L1 --box 1 43 2 44 --start 2021-03-01 --end 2021-03-31