Add categorization fields usage docs

[ebeahan]

Summary

Adds a new documentation section, Using the Categorization Fields, that contains guidance for using the four ECS categorization fields together to classify real-world example events.

Motivation

Help those implementing ECS to use the event categorization fields more consistently.

Design

While originally considered adding fields into the field definitions to provide examples, the approach felt too restrictive to capture more subtle or nuanced context. I instead opted for a new page, docs/field-values-usage.asciidoc, with an include directive to add the new content to the existing ECS Categorization Fields section.

After the section introduction, I captured some of the categorization usage principles from the public ECS categorization doc. This general guidance seemed like a good addition alongside the following examples.

Next are the actual examples. Each example follows this structure:

• Intro
• Code block with callouts
• Callout explanations
• Optionally, additional description or usage guidance relating to the example.

Docs Preview

Closes #860

[leehinman]

This is awesome.

comments are all suggestions/options except for the event.kind vs event.type one

[MikePaquette]

Looks awesome @ebeahan !
Thanks for adding this section.

I commented two minor grammar items, but I'm approving with a nice LGTM.

cc: @epixa