Improve Issue Templates (#5836)

Be explicit about searching for an existing `bug`, `feature`, `enhancement`
and avoid duplications.

Co-authored-by: Manuel de la Peña <social.mdelapenya@gmail.com>

.github/ISSUE_TEMPLATE/bug_report.yaml

@@ -6,7 +6,7 @@ body:
   - type: markdown
     attributes:
       value: |
-        Thanks for taking the time to fill out this bug report!
+        Thanks for taking the time to fill out this bug report! Before submitting a `bug`, please make sure there is no existing issue for the one you encountered.
   - type: dropdown
     id: module
     attributes:

.github/ISSUE_TEMPLATE/enhancement.yaml

@@ -6,7 +6,7 @@ body:
   - type: markdown
     attributes:
       value: |
-        Please provide the following information
+        Before submitting an `enhancement`, please make sure there is no existing enhancement for the one you are requesting. If so, please provide the following information:
   - type: dropdown
     id: module
     attributes:

.github/ISSUE_TEMPLATE/feature.yaml

@@ -6,7 +6,7 @@ body:
   - type: markdown
     attributes:
       value: |
-        Please provide the following information
+        Before submitting a `feature`, please make sure there is no existing feature for the one you are requesting. If so, please provide the following information:
   - type: dropdown
     id: module
     attributes: