Add concept headings to introduction.md (exercism#2713)

Author: ErikSchierboom

exercises/concept/chaitanas-colossal-coaster/.docs/introduction.md

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+## list-methods
+
 Python allows you to manipulate `list` in a lot of ways. A `list` is simple a collection of objects. They are mutable, ordered and indexed. Let's look at the methods that are available to manipulate a `list` object.
 
 When you manipulate a list with a list-method, you are changing the properties of the list you pass. That is, **you will alter the list** object that is being used with the list-method. If you do not want to change the original list, you need to copy the list and then work on the copied list.

exercises/concept/ghost-gobble-arcade-game/.docs/introduction.md

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+## bools
+
 Python represents true and false values with the `bool` type. There are only two values: `True` and `False`. These values can be bound to a variable:
 
 ```python

exercises/concept/guidos-gorgeous-lasagna/.docs/introduction.md

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+## basics
+
 Python is a dynamic and strongly typed object-oriented programming language. It supports both imperative (_object-oriented, procedural_) and declarative (_functional, concurrent_) programming paradigms. Python emphasizes code readability and (similar to Haskell) uses significant whitespace.
 
 Objects are assigned to names in Python via the `=` or _assignment operator_. _Variables_ are written in [`snake_case`](https://en.wikipedia.org/wiki/Snake_case), and _constants_ (usually) in `SCREAMING_SNAKE_CASE`. A name (_variable or constant_) is not itself _typed_, and can be attached or re-attached to different objects over its lifetime:

exercises/concept/inventory-management/.docs/introduction.md

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+## dicts
+
 A _dictionary_ is Python's primary mapping type that connects _hashable keys_ with values. The looking up of keys is more efficient than searching through an array, but does require more memory.
 
 ## Dict construction

exercises/concept/log-levels/.docs/introduction.md

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+## enums
+
 In Python, an enum is a set of names that are bound to unique `literal`, or `constant` values. Enums are defined the way classes are defined in python.
 
 ```python

exercises/concept/making-the-grade/.docs/introduction.md

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+## loops
+
 ## Loops in Python
 
 There are 2 general ways in Python to loop through objects.

exercises/concept/processing-logs/.docs/introduction.md

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+## strings
+
 A `str` typed object in Python is an immutable sequence of Unicode code points. This could include letters, numbers, punctuation, etc. To manipulate strings, Python provides string methods that can transform a string into other types, create new strings based on method arguments, or return information about the string. Strings can be concatenated with `+`.
 
 Immutability means that a string's value doesn't change; methods that appear to modify a string actually return a new instance of `str`.

exercises/concept/restaurant-rozalynn/.docs/introduction.md

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+## none
+
 In Python, `None` is frequently used to represent the absence of a value -- a placeholder to define a `null` (empty) variable, object, or argument.
 
 If you've heard about or used a `NULL` or `nil` type in another programming language, then this usage of `None` in Python will be familiar to you. `None` helps you to declare variables or function arguments that you don't yet have values for. These can then be re-assigned to specific values later as needed.

exercises/concept/tisbury-treasure-hunt/.docs/introduction.md

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+## tuples
+
 In Python, a [tuple](https://docs.python.org/3/library/stdtypes.html#tuple) is an immutable collection of items in _sequence_. Like most collections, tuples can hold any (or multiple) data type(s) -- including other tuples. Like any sequence, items are referenced by 0-based index number. Tuples support all [common sequence operations](https://docs.python.org/3/library/stdtypes.html#common-sequence-operations).
 
 ## Tuple Construction