[v0.18.0] New selectors

[jtcohen6]

Document new node selector functionality ahead of 0.18.0b2:

• methods: config, test_type, test_name, package
• intersections
• nth-parent/child
• version-controlled YML selectors

todos

• Merge Feature/yaml selections dbt-core#2640
• Release 0.18.0b2 I've decided I don't need to wait for this
• Ensure example code is legit, especially selectors yml

[jtcohen6]

@clrcrl The goal here is to document new features coming in v0.18.0, most of which is around model selection, in the hopes of releasing an early beta version. I'm open to your thoughts on how to signpost prerelease-only functionality.