docs: Add contributing guidelines to PR and issue templates (#2682)

Author: blakeli0

.github/ISSUE_TEMPLATE/bug_report.md

@@ -6,7 +6,7 @@ about: Create a report to help us improve
 
 Thanks for stopping by to let us know something could be better!
 
-**PLEASE READ**: If you have a support contract with Google, please create an issue in the [support console](https://cloud.google.com/support/) instead of filing on GitHub. This will ensure a timely response.
+**PLEASE READ**: Before creating a new issue, please read our [contributing guidelines](https://github.com/googleapis/gapic-generator-java/blob/main/CONTRIBUTING.md). If you have a support contract with Google, please create an issue in the [support console](https://cloud.google.com/support/) instead of filing on GitHub. This will ensure a timely response.
 
 Please run down the following list and make sure you've tried the usual "quick fixes":
 

.github/ISSUE_TEMPLATE/feature_request.md

@@ -6,7 +6,8 @@ about: Suggest an idea for this library
 
 Thanks for stopping by to let us know something could be better!
 
-**PLEASE READ**: If you have a support contract with Google, please create an issue in the [support console](https://cloud.google.com/support/) instead of filing on GitHub. This will ensure a timely response.
+**PLEASE READ**: Before creating a new issue, please read our [contributing guidelines](https://github.com/googleapis/gapic-generator-java/blob/main/CONTRIBUTING.md).
+ If you have a support contract with Google, please create an issue in the [support console](https://cloud.google.com/support/) instead of filing on GitHub. This will ensure a timely response.
 
 **Is your feature request related to a problem? Please describe.**
 What the problem is. Example: I'm always frustrated when [...]

.github/PULL_REQUEST_TEMPLATE.md

@@ -1,7 +1,6 @@
-Thank you for opening a Pull Request! For general contributing guidelines, please refer to [contributing guide](https://github.com/googleapis/gapic-generator-java/blob/main/CONTRIBUTING.md)
-
-Before submitting your PR, there are a few things you can do to make sure it goes smoothly:
+Thank you for opening a Pull Request! Before submitting your PR, please read our [contributing guidelines](https://github.com/googleapis/gapic-generator-java/blob/main/CONTRIBUTING.md). 
 
+There are a few things you can do to make sure it goes smoothly:
 - [ ] Make sure to open an issue as a [bug/issue](https://github.com/googleapis/gapic-generator-java/issues/new/choose) before writing your code! That way we can discuss the change, evaluate designs, and agree on the general idea
 - [ ] Ensure the tests and linter pass
 - [ ] Code coverage does not decrease (if any source code was changed)