Align the PR description with dbt-core, dbt-adapters, etc.

[dbeatty10]

resolves #242

Problem

The PR template of dbt-codegen has a single section for the main body of the description:

• Description & motivation

But dbt-core, dbt-adapters, etc. have two sections:

• Problem
• Solution

The latter allows the contributor to separate the design decisions of the implementation from the problem that it is trying to solve. This makes it easier for the contributor to write the PR description. It also makes it easier for others to read (code reviewers, users, etc.)

Solution

Adopt the standard established by dbt-core, dbt-adapters, etc in order to make it easier to read/write and also for those working across repos.

Checklist

• This code is associated with an issue which has been triaged and accepted for development.
• I have read the contributing guide and understand what's expected of me
• [N/A] I have verified that these changes work locally
• [N/A] I have updated the README.md (if applicable)
• [N/A] I have added tests & descriptions to my models (and macros if applicable)