Add nullish coalescing to multiple articles, refactor operators, renumber the chapter

1-js/02-first-steps/08-operators/1-increment-order/solution.md

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+
+The answer is:
+
+- `a = 2`
+- `b = 2`
+- `c = 2`
+- `d = 1`
+
+```js run no-beautify
+let a = 1, b = 1;
+
+alert( ++a ); // 2, prefix form returns the new value
+alert( b++ ); // 1, postfix form returns the old value
+
+alert( a ); // 2, incremented once
+alert( b ); // 2, incremented once
+```
+