Merge pull request #5749 from ethereum/docs-split-abstract

[DOCS] Split Abstract Contracts docs to new file

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+.. index:: ! contract;abstract, ! abstract contract
+
+.. _abstract-contract:
+
+******************
+Abstract Contracts
+******************
+
+Contracts are marked as abstract when at least one of their functions lacks an implementation as in the following example (note that the function declaration header is terminated by ``;``)::
+
+    pragma solidity >=0.4.0 <0.6.0;
+
+    contract Feline {
+        function utterance() public returns (bytes32);
+    }
+
+Such contracts cannot be compiled (even if they contain implemented functions alongside non-implemented functions), but they can be used as base contracts::
+
+    pragma solidity >=0.4.0 <0.6.0;
+
+    contract Feline {
+        function utterance() public returns (bytes32);
+    }
+
+    contract Cat is Feline {
+        function utterance() public returns (bytes32) { return "miaow"; }
+    }
+
+If a contract inherits from an abstract contract and does not implement all non-implemented functions by overriding, it will itself be abstract.
+
+Note that a function without implementation is different from a :ref:`Function Type <function_types>` even though their syntax looks very similar.
+
+Example of function without implementation (a function declaration)::
+
+    function foo(address) external returns (address);
+
+Example of a Function Type (a variable declaration, where the variable is of type ``function``)::
+
+    function(address) external returns (address) foo;
+
+Abstract contracts decouple the definition of a contract from its implementation providing better extensibility and self-documentation and
+facilitating patterns like the `Template method <https://en.wikipedia.org/wiki/Template_method_pattern>`_ and removing code duplication.
+Abstract contracts are useful in the same way that defining methods in an interface is useful. It is a way for the designer of the abstract contract to say "any child of mine must implement this method".