Documentation Gaps and Improvements

In addition to the documentation updates captured in #142, #182 and #640, there are other improvements that could be made. This issue is being created to capture the known issues. Depending on the final solution identified, this issue may need to be split into several smaller issues for implementation.

**General**
- Consider changing default value for `init_woodwork` in `demo.load_retail` to `False` as this might make the examples a little cleaner, when we are using this function to load a DataFrame
- Create a Woodwork Glossary of Terms

**Understanding Types and Tags Guide**
- Include mention of `list_semantic_tags` util function

**Gain Statistical Insights Guide**
- Include mention of `get_valid_mi_types` util function
- We should have a place in our documentation that mentions why Mutual Information is powerful measure, the types that it supports, and how its better than Pearson Correlation. (From #354)

**Other Topics that Should be Included**
- Selecting data with `iloc` and `loc`
- Setting and getting data from a dataframe
- Methods in Woodwork that overwrite pandas methods (`pop`, `drop`, `rename`)
- Setting and using metadata
- Column description attribute
- Understanding the Schema - what is it, when it is valid, what causes invalidation and what to do if schema is invalidated
- Serialization/Deserialization

## Possible Improvements/New Guides
- Split current Types and Tags guide into two separate guides, one providing an overview of Types and Tags, and one more focused on setting and changing types and tags
- About Types and Tags
- explain types and tags - even explain how certain logical types have validity checks or formatting requirements
- explain how all of these work together to build Woodwork’s type system - inference functions!!
- can be used as a simple cheat sheet if you’re trying to understand Woodwork language
- reference the customizability of the type system
- Using Types and Tags
- explain how types and tags are inferred
- explain Woodwork indices
- explains how to set and update these values
- show example of Woodwork operations
- Shows how to use series’ as well
- explains how the typing info can be invalidated - shows examples with pandas operations
- Maybe create a detailed dataframe/series guide that covers how to do various operations:
- Setting/updating logical types
- Setting/updating semantic tags
- Selecting data (getitem options, iloc, loc, `select` method)
- Assigning new data with and without typing information (setitem)
- Create a Woodwork Glossary of Terms
- physical types
- logical types
- semantic tags
- dtype
- primary_dtype
- backup_dtype
- Table/Column Accessor
- schema
- Woodwork typing information
- Type System
- metadata
- column descriptions