Align the PR description with dbt-core, dbt-adapters, etc. (#244)

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

* Add hyperlink to the open issues

Author: dbeatty10

.github/pull_request_template.md

@@ -1,12 +1,22 @@
 resolves #
 
-## Description & motivation
+### Problem
+
+<!---
+  Describe the problem this PR is solving. What is the application state
+  before this PR is merged?
+-->
+
+### Solution
+
 <!---
-Describe your changes, and why you're making them.
+  Describe the way this PR solves the above problem. Add as much detail as you
+  can to help reviewers understand your changes. Include any alternatives and
+  tradeoffs you considered.
 -->
 
 ## Checklist
-- [ ] This code is associated with an issue which has been triaged and [accepted for development](https://docs.getdbt.com/docs/contributing/oss-expectations#pull-requests). 
+- [ ] This code is associated with an [issue](https://github.com/dbt-labs/dbt-codegen/issues) which has been triaged and [accepted for development](https://docs.getdbt.com/docs/contributing/oss-expectations#pull-requests).
 - [ ] I have verified that these changes work locally
 - [ ] I have updated the README.md (if applicable)
 - [ ] I have added tests & descriptions to my models (and macros if applicable)