x/website: add more books

[mark-summerfield]

Page:

https://go.dev/learn/

In the books section there is no mention of these two books:

• "Learning Go" https://www.amazon.com/Learning-Go-Idiomatic-Real-World-Programming/dp/1492077216/
• "Programming in Go" https://www.amazon.com/Programming-Go-Creating-Applications-Developers/dp/0321774639/

"Learning Go" is really excellent so that one should definitely be on the page.
My "Programming in Go" is good too, but old now.

[dr2chase]

@golang/tools-team

[findleyr]

CC @ansaba, who is looking into this content.

(Saba: is there a better way to forward along this suggestion?)