doc: Bring back issue templates and update them

.gitlab/issue_templates/Bug.md

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+<!-- Please note that Fractal was rewritten from scratch and our current focus is on the main branch
+or the latest nightly build, so issues with the stable release are unlikely to be fixed. -->
+
+Detailed description of the issue. Provide as much information as you can, potentially with images
+or videos showing the issue.
+
+## Steps to reproduce
+
+1. Open Fractal
+2. Enter a room
+3. ...
+
+## Information
+
+* [ ] This bug is reproducible from the latest nightly build <!-- Check this box if the bug happens on Fractal's development version -->
+* **Fractal Version**: <!-- The version of Fractal you were using when the bug occurred. Check the "About Fractal" dialog for this information -->
+* **OS Version**: <!-- Operating system version, e.g. Fedora 36 -->
+* **Installation Source**: <!-- Where you installed Fractal from, e.g. Flathub, GNOME Apps Nightly, AUR, or distro repositories -->
+* **Homeserver**: <!-- The homeserver for your matrix account, e.g. matrix.org, gnome.org, … You can mention several of them if this is reproducible on multiple ones. -->
+
+<!-- If you have a crash report, use the "Attach A File" button in the issue editor to attach it, or paste it in a code block below. -->
+<!-- To create a code block, you surround the code with two lines with three "`" characters like so:
+
+```
+code goes here
+```
+
+-->

.gitlab/issue_templates/Feature.md

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+<!-- Please note that Fractal was rewritten from scratch and our current focus is on the main branch
+or the latest nightly build. Some features missing in the stable release are already available
+in the development version. To avoid duplicates and unnecessary issues, please check that your
+request is for something that is not yet implemented and doesn’t have an existing issue open. -->
+
+Detailed description of the feature. Provide as much information as you can.
+
+Proposed Mockups:
+
+(Add mockups of the proposed feature)
+
+## Design Tasks
+
+* [ ]  design tasks
+
+## Development Tasks
+
+* [ ]  development tasks
+
+## QA Tasks
+
+* [ ]  quality assurance tasks