Feature/mkdocs with versioning (#333)

* docs: restructuring & introduction of mkdocs with versioning provided by mike

* docs: ad repositories section to sophia examples

* docs: refactoring and consistent naming of æternity

* docs: hint for new file destination

* docs: revert capital letter

* docs: accept proposed changes

* docs: fix anchors in stdlib

docs/index.md

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+# Introduction
+Sophia is a functional language designed for smart contract development. It is strongly typed and has
+restricted mutable state.
+
+Sophia is customized for smart contracts, which can be published
+to a blockchain. Thus some features of conventional
+languages, such as floating point arithmetic, are not present in Sophia, and
+some [æternity blockchain](https://aeternity.com) specific primitives, constructions and types have been added.
+
+!!! Note
+    - For rapid prototyping of smart contracts check out [AEstudio](https://studio.aepps.com/)!
+    - For playing around and diving deeper into the language itself check out the [REPL](https://repl.aeternity.io/)!