Update PR Contributor Checklist (#8255)

# Description

The current checklist doesn't provide a clear checkbox for scenarios
where individual items are not applicable. To make it explicit, adding
two checkbox options on each of the items - "Yes" and "Not Applicable".
Based on the PR, one of the two options should be checked.

## Type of change

Fixes: #8244 

## Contributor checklist
Please verify that the PR meets the following requirements, where
applicable:

- [NA] An overview of proposed schema changes is included in a linked
GitHub issue.
- [NA] A design document PR is created in the [design-notes
repository](https://github.com/radius-project/design-notes/), if new
APIs are being introduced.
- [NA] If applicable, design document has been reviewed and approved by
Radius maintainers/approvers.
- [NA] A PR for the [samples
repository](https://github.com/radius-project/samples) is created, if
existing samples are affected by the changes in this PR.
- [NA] A PR for the [documentation
repository](https://github.com/radius-project/docs) is created, if the
changes in this PR affect the documentation or any user facing updates
are made.
- [NA] A PR for the [recipes
repository](https://github.com/radius-project/recipes) is created, if
existing recipes are affected by the changes in this PR.

Signed-off-by: Karishma Chawla <[email protected]>

.github/pull_request_template.md

@@ -27,9 +27,21 @@ Fixes: #issue_number
 ## Contributor checklist
 Please verify that the PR meets the following requirements, where applicable:
 
-- [ ] An overview of proposed schema changes is included in a linked GitHub issue.
-- [ ] A design document PR is created in the [design-notes repository](https://github.com/radius-project/design-notes/), if new APIs are being introduced.
-- [ ] If applicable, design document has been reviewed and approved by Radius maintainers/approvers.
-- [ ] A PR for the [samples repository](https://github.com/radius-project/samples) is created, if existing samples are affected by the changes in this PR.
-- [ ] A PR for the [documentation repository](https://github.com/radius-project/docs) is created, if the changes in this PR affect the documentation or any user facing updates are made.
-- [ ] A PR for the [recipes repository](https://github.com/radius-project/recipes) is created, if existing recipes are affected by the changes in this PR.
+- An overview of proposed schema changes is included in a linked GitHub issue.
+    - [ ] Yes
+    - [ ] Not applicable
+- A design document PR is created in the [design-notes repository](https://github.com/radius-project/design-notes/), if new APIs are being introduced.
+    - [ ] Yes
+    - [ ] Not applicable
+- The design document has been reviewed and approved by Radius maintainers/approvers.
+    - [ ] Yes
+    - [ ] Not applicable
+- A PR for the [samples repository](https://github.com/radius-project/samples) is created, if existing samples are affected by the changes in this PR.
+    - [ ] Yes
+    - [ ] Not applicable
+- A PR for the [documentation repository](https://github.com/radius-project/docs) is created, if the changes in this PR affect the documentation or any user facing updates are made.
+    - [ ] Yes
+    - [ ] Not applicable
+- A PR for the [recipes repository](https://github.com/radius-project/recipes) is created, if existing recipes are affected by the changes in this PR.
+    - [ ] Yes
+    - [ ] Not applicable