Sync pangram docs with problem-specifications (exercism#1638)

The pangram exercise has 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, as well as
the pull request that updated the exercise in the problem-specifications
repository:

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

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:
-> The quick brown fox jumps over the lazy dog.
+Your task is to figure out if a sentence is a pangram.
 
-The alphabet used consists of ASCII letters `a` to `z`, inclusive, and is case
-insensitive. Any characters which are not an ASCII letter should be ignored.
+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`).
+
+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.
+```