update loops concept docs

[oxe-i]

I'm using two LLMs (Claude and Gemini) to review every concept in the syllabus for typos and conceptual gaps. Two suggestions seemed interesting:

1. The loops concept emphasizes the loop instructions, but they are rarely used in practice because they are slow. I confirmed this against Agner Fog's instruction tables, a classic reference. Only a few AMD CPUs support them with a reasonable speed.
2. The recursion concept glosses over the stack frame, without going into practical concerns such as saving variables on the stack and using a prologue to keep track of each call frame.

This PR addresses the first suggestion.

I removed the section on the loop instructions from the loops concept's introduction.md, but kept it in about.md, now with a note explaining that these instructions are slow.

While there, I added two notes to introduction.md: one on using instructions other than cmp to set flags (a student coming from the conditionals concept would be using cmp for everything), and an advanced note on macrofusion. Both topics are explored in more depth in about.md.