Migrate documentation to VuePress

GitBook development seems a bit stuck right now so let's switch to VuePress which, IMO, is more user friendly. Update the documentation to include the version number in which features were added and use custom container to display notes and warnings.

docs/.vuepress/config.js

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+module.exports = {
+    title: 'QtPromise',
+    description: 'Promises/A+ implementation for Qt/C++',
+    ga: 'UA-113899811-1',
+    head: [
+        ['link', { rel: 'icon', href: `/favicon.png` }],
+    ],
+    themeConfig: {
+        repo: 'simonbrunel/qtpromise',
+        lastUpdated: 'Last Updated',
+        editLinks: true,
+        docsDir: 'docs',
+        sidebar: [
+            'qtpromise/getting-started',
+            'qtpromise/qtconcurrent',
+            'qtpromise/thread-safety',
+            'qtpromise/api-reference',
+            {
+                title: 'QPromise',
+                children: [
+                    'qtpromise/qpromise/constructor',
+                    'qtpromise/qpromise/delay',
+                    'qtpromise/qpromise/each',
+                    'qtpromise/qpromise/fail',
+                    'qtpromise/qpromise/filter',
+                    'qtpromise/qpromise/finally',
+                    'qtpromise/qpromise/isfulfilled',
+                    'qtpromise/qpromise/ispending',
+                    'qtpromise/qpromise/isrejected',
+                    'qtpromise/qpromise/map',
+                    'qtpromise/qpromise/tap',
+                    'qtpromise/qpromise/tapfail',
+                    'qtpromise/qpromise/then',
+                    'qtpromise/qpromise/timeout',
+                    'qtpromise/qpromise/wait',
+                    'qtpromise/qpromise/all.md',
+                    'qtpromise/qpromise/reject.md',
+                    'qtpromise/qpromise/resolve.md'
+                ]
+            },
+            {
+                title: 'Helpers',
+                children: [
+                    'qtpromise/helpers/attempt',
+                    'qtpromise/helpers/each',
+                    'qtpromise/helpers/filter',
+                    'qtpromise/helpers/map',
+                    'qtpromise/helpers/qpromise',
+                    'qtpromise/helpers/qpromiseall'
+                ]
+            }
+        ]
+    }
+}

docs/.vuepress/override.styl

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+$accentColor = #23b223
+$textColor = #404244

docs/.vuepress/style.styl

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+@import 'override.styl'
+
+.content a code
+    color: $accentColor