various docs updates

docs/reference/logic.md

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+# Logic
+
+[if](/microbit/reference/logic/if)
+
+```blocks
+if(true) {
+}
+```
+
+[Boolean](/microbit/reference/types/boolean) values: *true*; *false*
+
+```blocks
+true
+false
+```
+
+Boolean binary operators: *and* (conjunction); *or* (disjunction)
+
+```blocks
+true && false;
+true || false;
+```
+
+Boolean negation operator
+
+```blocks
+!true
+```
+
+Comparison operators (=, !=, <, >, <=, >=)
+
+```blocks
+0 == 0;
+1 !- 0;
+0 < 1;
+1 > 0;
+0 <= 1;
+1 >= 0;
+```

docs/reference/loops.md

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+# Loops
+
+Repeat code.
+
+
+[for](/reference/loops/for)
+
+```blocks
+for(let i = 0;i<5;i++) {}
+```
+
+[repeat](/reference/loops/repeat)
+
+![](/static/mb/blocks/contents-0.png)
+
+[while](/reference/loops/while)
+
+```blocks
+while(true) {}
+```
+
+[forever](/reference/basic/forever)
+
+```blocks
+basic.forever(() => {})
+```

docs/reference/math.md

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+# Math
+
+[Numeric](/reference/types/number) values: 0, 1, 2, ...
+
+```blocks
+0;
+1;
+2;
+```
+
+Arithmetic binary operation (+, -, *, /)
+
+```blocks
+0+1;
+0-1;
+1*2;
+3/4;
+```
+
+Absolute value
+
+```blocks
+Math.abs(-5);
+```
+
+Minimum/maximum of two values
+
+```blocks
+Math.min(0, 1);
+Math.max(0, 1);
+```
+
+Random value
+
+```blocks
+Math.random(5);
+```

docs/reference/variables.md

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+## Variables
+
+[Assign](/reference/variables/assign) (set) a variable's value
+
+```blocks
+let x = 0;
+```
+
+Get a variable's value
+
+```blocks
+let x = 0;
+x;
+```
+
+[Change](/reference/variables/change-var) a variable's value
+
+```blocks
+let x = 0;
+x+=1;
+```