Upgrade documentation site

docs/usage/advanced.md

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+---
+title: Advanced usage
+---
+
+## Alphabet as a string
+
+```js
+alphabet.join('')[0];
+//=> 'abcdefghijklmnopqrstuvwxyz'
+```
+
+## Uppercase
+
+```js
+alphabet.toUpperCase();
+//=> ["Α", "Β", "Γ", "Δ", "Ε", "Ζ", "Η", "Θ", "Ι", "Κ", "Λ", "Μ", "Ν", "Ξ", "Ο", "Π", "Ρ", "Σ", "Τ", "Υ", "Φ", "Χ", "Ψ", "Ω"]
+```

docs/usage/installation.md

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+---
+title: Installation
+---
+
+## npm
+
+```bash
+npm install alphabets --save
+```
+
+## Yarn
+
+```bash
+yarn add alphabets
+```
+
+## CDN
+
+```html
+<script src="https://unpkg.com/alphabet@1/dist/bundle.js"></script>
+```

docs/usage/usage.md

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+---
+title: Usage
+toc: false
+---
+
+```js
+import { <alphabetYouWantToUse> } from 'alphabets';
+```
+
+or:
+
+```js
+const alphabets = require('alphabets');
+console.log(alphabets.<alphabetYouWantToUse>);
+```
+
+where `<alphabetYouWantToUse>` is an alphabet this package supports.
+[Here](/api/supported-alphabets) is a list from which you can choose.