[Doc] Documentation of Linearizable and their methods e.g, get_text(config)

```Document``` class has ```get_text(config: TextLinearizationConfig)``` method as in the example [Using Layout Analysis for Text Linearization](https://aws-samples.github.io/amazon-textract-textractor/notebooks/layout_analysis_for_text_linearization.html) cell 19.

```
from textractor.data.text_linearization_config import TextLinearizationConfig

config = TextLinearizationConfig(
hide_figure_layout=True,
title_prefix="# ",
section_header_prefix="## "
)
print(document.get_text(config=config)) # <--- get_text() method
```

However, it looks the documentation only has [get_text_and_words](https://aws-samples.github.io/amazon-textract-textractor/textractor.entities.html#textractor.entities.document.Document.get_text_and_words) method but it does not have ```get_text``` which is the method of the parent class [Linearizable(ABC): ](https://github.com/aws-samples/amazon-textract-textractor/blob/master/textractor/entities/linearizable.py#L13-#23).

It would be desirable to have a clear definition and explanation of what ```Linearizable``` and what methods it has, it is being used in the sample codes, rather than going through the github code to verify what it is.