exercises(pangram): sync docs (#497)

The pangram docs have been overhauled as part of a project to make
practice exercises more consistent and friendly.

For more context, please see the discussion in the forum [1], as well as
the pull request that updated the exercise in the problem-specifications
repository [2].

[1] https://forum.exercism.org/t/new-project-making-practice-exercises-more-consistent-and-human-across-exercism/3943
[2] exercism/problem-specifications#2215

Author: kytrinyx

exercises/practice/pangram/.docs/instructions.md

@@ -1,9 +1,8 @@
 # Instructions
 
-Determine if a sentence is a pangram.
-A pangram (Greek: παν γράμμα, pan gramma, "every letter") is a sentence using every letter of the alphabet at least once.
-The best known English pangram is:
+Your task is to figure out if a sentence is a pangram.
 
-> The quick brown fox jumps over the lazy dog.
+A pangram is a sentence using every letter of the alphabet at least once.
+It is case insensitive, so it doesn't matter if a letter is lower-case (e.g. `k`) or upper-case (e.g. `K`).
 
-The alphabet used consists of letters `a` to `z`, inclusive, and is case insensitive.
+For this exercise we only use the basic letters used in the English alphabet: `a` to `z`.

exercises/practice/pangram/.docs/introduction.md

@@ -0,0 +1,16 @@
+# Introduction
+
+You work for a company that sells fonts through their website.
+They'd like to show a different sentence each time someone views a font on their website.
+To give a comprehensive sense of the font, the random sentences should use **all** the letters in the English alphabet.
+
+They're running a competition to get suggestions for sentences that they can use.
+You're in charge of checking the submissions to see if they are valid.
+
+```exercism/note
+Pangram comes from Greek, παν γράμμα, pan gramma, which means "every letter".
+
+The best known English pangram is:
+
+> The quick brown fox jumps over the lazy dog.
+```