PEMDAS is never defined in Part 1 of Wolfram Language article series, link broken

[rljacobson]

While PEMDAS has its own separate article, it should really be defined/explained in Part 1 of the Wolfram Language series of articles.

This problem is amplified by the fact that the link to the PEMDAS article is broken.

• Explain/define PEMDAS
• Fix link

(H/t Rocky)