minesweeper: split into introduction and instructions

exercises/minesweeper/instructions.md

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+# Instructions
+
+Your task is to add the mine counts to empty squares in a completed Minesweeper board.
+The board itself is a rectangle composed of squares that are either empty (`' '`) or a mine (`'*'`).
+
+For each empty square, count the number of mines adjacent to it (horizontally, vertically, diagonally).
+If the empty square has no adjacent mines, leave it empty.
+Otherwise replace it with the adjacent mines count.
+
+For example, you may receive a 5 x 4 board like this (empty spaces are represented here with the '·' character for display on screen):
+
+```text
+·*·*·
+··*··
+··*··
+·····
+```
+
+Which your code should transform into this:
+
+```text
+1*3*1
+13*31
+·2*2·
+·111·
+```

exercises/minesweeper/introduction.md

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+# Introduction
+
+[Minesweeper][wikipedia] is a popular game where the user has to find the mines using numeric hints that indicate how many mines are directly adjacent (horizontally, vertically, diagonally) to a square.
+
+[wikipedia]: https://en.wikipedia.org/wiki/Minesweeper_(video_game)