gitlab: add a gitlab issue template for bug

To clarify what is expected in an issue description and avoid
issue which is just an usage issue, add a bug template in gitlab.

Part-of: <https://gitlab.freedesktop.org/gstreamer/gst-build/-/merge_requests/251>

.gitlab/issue_templates/Bug.md

@@ -0,0 +1,32 @@
+### Describe your issue
+<!-- a clear and concise summary of the bug. -->
+<!-- For any GStreamer usage question, please contact the community using the #gstreamer channel on IRC https://www.oftc.net/ or the mailing list on https://gstreamer.freedesktop.org/lists/ -->
+
+#### Expected Behavior
+<!-- What did you expect to happen -->
+
+#### Observed Behavior
+<!-- What actually happened -->
+
+#### Setup
+- **Operating System:**
+- **Device:** Computer / Tablet / Mobile / Virtual Machine <!-- Delete as appropriate !-->
+- **GStreamer Version:**
+- **Command line:**
+
+### Steps to reproduce the bug
+<!-- please fill in exact steps which reproduce the bug on your system, for example: -->
+1. open terminal
+2. type `command`
+
+### How reproducible is the bug?
+<!-- The reproducibility of the bug is Always/Intermittent/Only once after doing a very specific set of steps-->
+
+### Screenshots if relevant
+
+### Solutions you have tried
+
+### Related non-duplicate issues
+
+### Additional Information
+<!-- Any other information such as logs. Make use of <details> for long output -->