Rework two-fer documentation (#2204)

* Rework two-fer documentation

This splits two-fer into separate introduction and instructions
documents.

It also rewords the exercise to feel a bit less stilted, and
(hopefully) a bit more friendly. It adds a bit of explanation
for the definition of the term two-fer and where it comes from,
as well as providing a bit of context ("story", if you will) for
the exercise itself.

* Update exercises/two-fer/introduction.md

Co-authored-by: ee7 <45465154+ee7@users.noreply.github.com>

* Tweak two-fer docs

Give a less socially awkward reason for saying 'you'.

* Update exercises/two-fer/instructions.md

Co-authored-by: ee7 <45465154+ee7@users.noreply.github.com>

---------

Co-authored-by: ee7 <45465154+ee7@users.noreply.github.com>

Author: kytrinyx

exercises/two-fer/description.md

@@ -0,0 +1,25 @@
+# Instructions
+
+Your task is to determine what you will say as you give away the extra cookie.
+
+If your friend likes cookies, and is named Do-yun, then you will say:
+
+```text
+One for Do-yun, one for me.
+```
+
+If your friend doesn't like cookies, you give the cookie to the next person in line at the bakery.
+Since you don't know their name, you will say _you_ instead.
+
+```text
+One for you, one for me.
+```
+
+Here are some examples:
+
+|Name    |Dialogue
+|:-------|:------------------
+|Alice   |One for Alice, one for me.
+|Bohdan  |One for Bohdan, one for me.
+|        |One for you, one for me.
+|Zaphod  |One for Zaphod, one for me.

exercises/two-fer/instructions.md

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+# Introduction
+
+In some English accents, when you say "two for" quickly, it sounds like "two fer".
+Two-for-one is a way of saying that if you buy one, you also get one for free.
+So the phrase "two-fer" often implies a two-for-one offer.
+
+Imagine a bakery that has a holiday offer where you can buy two cookies for the price of one ("two-fer one!").
+You go for the offer and (very generously) decide to give the extra cookie to a friend.