syntax, markdown and contents revision

[oxe-i]

Changelog:

1. Add observation on the Floating-Point Numbers concept to indicate that the comparison between them should be treated as unsigned.
2. Add observation on the Strings concept to indicate that a dereferenced memory location may be used directly as operands to most instructions, without first loading its contents in a register.
3. Add notes in the solution file for Inventory Management, to indicate the size of each argument and the return value.
4. Add a table to the Poetry Club Door Policy instructions with the size for the string instructions.
5. Since the Floating-Point Numbers concept requires the Memory concept, I've adjusted the exemplar solution to make use of constants and be more idiomatic.
6. Removed all links for the introduction.md in every concept exercise.

[oxe-i]

macOS complains about size mismatch if I use a 64-bit memory operand directly in a SSE instruction. I can't use a m64 operand (which is standard) and a m32 or m128 is an actual mismatch (so no assembler/compiler passes).

So, basically, the simulator forces the memory operand to be loaded first into a xmm register.