Link to contract programming page from functions spec.

So that readers will understand what the in and out blocks are for.

Fixes: https://issues.dlang.org/show_bug.cgi?id=13328
[Refactoring] Add non terminal symbol `FunctionContracts`
[Refactoring] Add non terminal symbol `FunctionLiteralBody` to separate grammar from normal functions
fix Issue 12741 - DMD accepts functions with contracts and no body

Support normal function declaration without body.
`UnitTest` cannot have contracts
Add code example for foreach over delegate.
Announce 2.066.1-rc2
fix Issue 10232 - AndExpression grammar is not correct
fix AndAndExpression grammar
Merge pull request #658 from quickfur/issue4954

Add code example for foreach over delegate.
Merge pull request #657 from 9rnsr/fix12741

Issue 12741 - DMD accepts functions with contracts and no body
fix Issue 13451 - Lambda syntax with explicit return type not documented
Merge pull request #654 from quickfur/issue13328

Link to contract programming page from functions spec.
Merge pull request #656 from 9rnsr/fix13451

Issue 13451 - Lambda syntax with explicit return type not documented
Merge pull request #659 from 9rnsr/fix10232

Issue 10232 - AndExpression grammar is not correct