[Elm] Add concept introductions

languages/elm/concepts/basics/introduction.md

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+### Constants and functions
+
+A constant value is defined with `name = expression`,
+where in Elm, everything except definitions are expressions.
+
+```elm
+five = 5
+
+six = 3 + 3
+```
+
+Functions are defined with `name parameters... = expression`,
+parameters being only separated by space.
+
+```elm
+add number1 number2 = number1 + number2
+```
+
+Invoking a function also is an expression and is done by
+specifying its name and passing arguments for each of the function's parameters,
+separated by space, just like for function definition.
+
+```elm
+five = add 2 3
+```
+
+Parentheses can be used to specify the order of evaluation of an expression.
+
+```elm
+six = add 2 (1 * 4)
+
+twelve = add 2 1 * 4
+```
+
+### Indentation / significant whitespace
+
+Elm doesn't use syntactic markers such as curly brackets, parentheses, or semicolons to specify code boundaries. It uses whitespaces and indentations instead.
+
+- Module, import, and top-level function definitions must start at the left most column.
+- If an expression is split into multiple lines, the code that is part of that expression must be indented under that expression with at least one space.
+- Parts of the expression that are grouped together should be indented with equal number of spaces.
+
+```elm
+-- A function split over multiple lines, so subsequent lines must be indented
+add number1 number2 =
+    number1 + number2
+```
+
+https://elmprogramming.com/indentation.html
+
+### Modules
+
+Each file in Elm is a module, and must contain a `module` statement before all other code.
+Module names must match their file name, so module `Calculator` must be in file Calculator.elm.
+Anything defined within a module is privatly scoped to it
+and cannot be accessed from outside this module, unless listed in `exposing`.
+
+```elm
+-- Define the Calculator module, and expose the `add` function
+module Calculator exposing (add)
+
+six = 3 + 3
+
+add number1 number2 = number1 + number2
+```
+
+```elm
+-- Define the Calculator module, and expose everything within: `six` and `add`
+module Calculator exposing (..)
+
+six = 3 + 3
+
+add number1 number2 = number1 + number2
+```
+
+https://elm-lang.org/docs/syntax#modules
+
+### Comments
+
+A comment is some text within the Elm file that is not interpreted as code.
+It is mainly intented to be read by yourself and other programmers.
+There is a lightweight syntax for single line comments, based on double dashes.
+Multiline comments are also possible with the `{-` and `-}` pair
+of opening and closing delimiters.
+
+```elm
+-- a single line comment
+-- another single line comment
+
+{- a multiline comment
+   spawning multiple lines
+-}
+```
+
+### Formatting
+
+There is a [style guide](https://elm-lang.org/docs/style-guide),
+and [elm-format](https://github.com/avh4/elm-format) can be used to automatically format code.