Minor miscellaneous improvements and edits

Some minor changes I noted could be made while reading through the book - not all may be suitable, but hopefully there at a few useful improvements.

* `foreword.md, applications-received.md` Punctuation
* `first-day.md, tailoring-cv.md` Replace mentions of "Part 2" with a clearer phrasing, as the book does not have a marked Part 2.
* `writing-covering-letter.md` Replace the "online portfolio" link, due to there being no such section, and add notes on website/covering letter content.