Add link to public prices of SRAM parts to cost of memory article

[jafingerhut]

This is the article that should be edited: https://github.com/jafingerhut/p4-guide/blob/master/docs/cost-of-high-speed-storage.md

Ben Lewis pointed out this public source for prices of SRAM parts: https://uk.farnell.com/w/c/semiconductors-ics/memory/sram/prl/results?sort=P_PRICE%7C1

There is probably a similar one in US dollars, too, but if not easily found, just use that one and the current US dollars / British pound exchange rate, and mention the exchange rate used in the article.

[jafingerhut]

https://stackoverflow.com/questions/4666728/why-is-the-size-of-l1-cache-smaller-than-that-of-the-l2-cache-in-most-of-the-pro/38549736#38549736

Another link from Ben Lewis, although from my quickly skimming over it, it appears to be more about the differences between L1 vs. L2 vs. L3 cache logic in modern processors, and why they each have different area and power costs from each other. That may interest some, but is probably a bit more in detail than I want to get into in my article. Referencing it for those interested in more details seems reasonable.