Closes JIRA:PRODDOCS-991

This PR:

• Clarifies the YAML naming convention guidance on the dbt_project.yml reference page so it no longer presents dashes as a blanket rule.

• The previous docs said multi-word resource types in dbt_project.yml always use dashes, but data_tests and unit_tests intentionally use underscores everywhere (including dbt_project.yml). That mismatch caused user confusion.

• Soften the dash guidance to “most” multi-word resource types

• Document the data_tests / unit_tests underscore exception with an example

• Keep the existing outside-dbt_project.yml underscore guidance

• Minor style cleanup on related “refer to” link wording (_plus-prefix snippet)
  Related: dbt-core#9201, dbt-core#9235

Checklist

• The changes in this PR meet the docs style guide/fundamentals required for all content.
• Applied the proper versioning rules if the content is for specific dbt version(s): (version a whole page or version a block of content
• The content in this PR requires a dbt release note, so I added one to the release notes page.).

🚀 Deployment available! Here are the direct links to the updated files:

• https://docs-getdbt-com-git-nfiann-globalrule-dbt-labs.vercel.app/reference/dbt_project.yml