spelling and phrasing improvements

Fix some typos that were reported in github issues. (Thanks to all that found these!)

Add ribbon diagrams to reference card

Add ribbon diagrams to tri-fold refcard
Change refcard boolean operators for better readability

Add green card (page 1)

Refactored some diagrams to start a proper green-card.

imm_j and imm_b can only contain even values

Correct the offset ranges for B-type and J-type instructions.

Improve LI and LA pseudoinstruction description

Change notation used for incrementing the immediate operands of the LUI and AUIPC instructions when used in the LI and LA pseudo instructions.

Propagate same into %hi, %lo, et. al.

IEEE-754 floating point clarification

Clean up the drawing of the floating point value box to show slugs where hex digits are aligned.
Clarify discussion on comparing magnitudes of IEEE-754 formatted values.

Update toolchain install notes in Appendix A

Install python before the GNU toolchain for GDB.
Added notes on building qemu.

Rephrase how lengths are determined for shift insns

Change wording and diagrams concerning which bits are used from length values used in shift instructions.

Correct mislabeled sign on sudtrahend

v0.12

Correct mislabeled sign on sudtrahend

Rewrite the discussion of big-endian and little-endian storage

More examples and color highlighting were added to the section discussing big-endian and little-endian values and the way they are stored in memory.