Blog edits

[gailweiss]

• update link to personal website, and make final links clickable
• move to input/output position framing (as opposed to key/query framing) where it seems clearer
• add elaborations and assumptions to examples
• fix some layer vs attention head notes
• some typos and grammar
• add link and note about tracr, the rasp compiler (and clarification that not necessarily all of rasp is compilable, much as we hope it is)
• reorder challenges to separate between those that directly tie into long addition and those that are more tangential