Change JOIN tutorial to focus more on explicit joins

The following documentation comment has been logged on the website:

Page: https://www.postgresql.org/docs/12/tutorial-join.html
Description:

The tutorial about joins makes the following statement about the explicit
JOIN operator:

> This syntax is not as commonly used as the one above

I think in 2020 this claim is no longer true, and I would love to see the
manual prefer the "modern" explicit JOIN operator rather than sticking to
the ancient implicit joins in the WHERE clause.