maint: Format ull request template headers

Use a better readable header level

No functional changes.

Author: EwoutH

.github/PULL_REQUEST_TEMPLATE/bug.md

@@ -1,16 +1,16 @@
-## Summary
+### Summary
 <!-- Provide a brief summary of the bug and its impact. -->
 
-## Bug / Issue
+### Bug / Issue
 <!-- Link to the related issue(s) and describe the bug. Include details like the context, what was expected, and what actually happened. -->
 
-## Implementation
+### Implementation
 <!-- Describe the changes made to resolve the issue. Highlight any important parts of the code that were modified. -->
 
-## Testing
+### Testing
 <!-- Detail the testing performed to verify the fix. Include information on test cases, steps taken, and any relevant results.
 
 If you're fixing the visualisation, add before/after screenshots. -->
 
-## Additional Notes
+### Additional Notes
 <!-- Add any additional information that may be relevant for the reviewers, such as potential side effects, dependencies, or related work.

.github/PULL_REQUEST_TEMPLATE/feature.md

@@ -1,16 +1,16 @@
-## Summary
+### Summary
 <!-- Provide a concise summary of the feature and its purpose. -->
 
-## Motive
+### Motive
 <!-- Explain the reasoning behind this feature. Include details on the problem it addresses or the enhancement it provides. -->
 
-## Implementation
+### Implementation
 <!-- Describe how the feature was implemented. Include details on the approach taken, important decisions made, and code changes. -->
 
-## Usage Examples
+### Usage Examples
 <!-- Provide code snippets or examples demonstrating how to use the new feature. Highlight key scenarios where this feature will be beneficial.
 
 If you're modifying the visualisation, add before/after screenshots. -->
 
-## Additional Notes
+### Additional Notes
 <!-- Add any additional information that may be relevant for the reviewers, such as potential side effects, dependencies, or related work. -->