Concept docs for "Numbers" (closes #600)

concepts/numbers/.meta/config.json

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+{
+  "blurb": "C++ has different types for numbers. Common are int for integers and double for decimals.",
+  "authors": [
+    "vaeng"
+  ],
+  "contributors": [
+  ]
+}

concepts/numbers/about.md

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+# About
+
+TODO: Include text from introduction.md when final.
+
+
+~~~~exercism/advanced
+## Casting
+Casting is the conversion from one type to another.
+If a `double` is needed, an `int` can be used in its place.
+The `5.0 / 2` example from above is handled by an implicit cast of `2` to `2.0`.
+A `double`, that is cast into an `int` might lose information.
+It is good practice to make these casts to another type T explicit via `static_cast<T>`.
+The old C-style cast via `(T)` is discouraged in modern C++, but should be known nonetheless.
+```cpp
+double pi{3.14};
+int about_2_times_pi_cpp{static_cast<int>(pi) * 2};
+// Old C-style cast:
+int about_2_times_pi_c{(int)pi * 2};
+```
+
+## Precision & Representation
+
+Numbers in C++ cannot be abitrarily big.
+`int` for example is represented by 32 bits.
+That is enough for a set of `2^32` numbers - roughly 4 billion.
+Including `0`, this sets the biggest and smallest `int` numbers as `-2^31` and `2^31 - 1`.
+
+Floating point numbers are usually implemented using 15 decimal places of precision, but will vary in representation based on the host system.
+~~~~

concepts/numbers/introduction.md

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+# Introduction
+The built-in number types in C++ can be divided into integers and floating points.
+Integers are whole numbers like `0`, `691`, or `-2`.
+Floating point numbers are numbers with a decimal point like `6.02214076`, `0.1`, or `-1.616`.
+
+## Integers
+The following example shows the declaration and initialization of four different variables 
+
+```cpp
+int m_morales{9241};              // base 10: 0-9
+int a_apaec{0x24CD};              // base 16: 0-9 and A-F
+int m_gargan{0b10010000011001};   // base  2: 0-1
+int b_reilly{022031};             // base  8: 0-7
+                                  // Leading with a 0 not the letter o.
+```
+When you assign a value to an `int` variable, you can do so directly with a literal.
+A literal is a hard-coded number like `9241`.
+There are different integer literals for several bases of the representation.
+Decimal integer literals are the most common and use the digits `0` to `9`.
+By adding a special prefix, like `0x`, it is possible to use other bases.
+The example above shows the number `9421` in its four representations and prefixes.
+All variables are initialized to the same value.
+
+For more details on the different representation systems, take a look at [a small tutorial][cpp_numerical_bases].
+
+You can use an apostrophe to separate digits for easier readability.
+`9'241` is the same as `0b0100'100'0001'1001` or `92'4'1`.
+
+## Floating-Point Numbers
+
+The floating-point literals come in two flavors. 
+In addition to the intuitive `0.0024` it is possible to use its scientific notation `2.4e-3`.
+The most common floating-point type is `double`.
+
+## Arithmetic
+
+C++ supports `+`, `-`, `*`, `/`, `(` and `)` and `%` to form expressions.
+The result from the operation between two integers is also an integer.
+`5 / 2` will return `2`.
+When one of the involved types is a floating-point type, the result will also be of a floating-point.
+`5.0 / 2` and `5 / 2.0` will return `2.5`.
+`%` is the remainder operator and will return the remainder of an integer division: `5%3` is `2`.
+
+[cpp_numerical_bases]: https://cplusplus.com/doc/hex/

concepts/numbers/links.json

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+[
+    {
+      "url": "https://en.cppreference.com/w/cpp/language/operator_arithmetic",
+      "description": "Reference for Arithmetic operators"
+    },
+    {
+      "url": "https://en.cppreference.com/w/cpp/language/auto",
+      "description": "Reference for the auto placeholder type specifier"
+    },
+    {
+      "url": "https://en.wikipedia.org/wiki/Double-precision_floating-point_format",
+      "description": "Explanation on the floating point format"
+    }
+  ]

config.json

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       "slug": "strings",
       "name": "Strings"
     },
+    {
+      "uuid": "de675a2c-6fd2-427d-93c1-4190eeffcd95",
+      "slug": "numbers",
+      "name": "Numbers"
+    },
     {
       "uuid": "e52453a5-4997-4eba-b754-5bda7b97c701",
       "slug": "basics",