Add initial task proposal issue template

Mostly from
- https://git.soi.ch/jan/proposal-experiment/src/branch/master/.gitea/issue_template/task-proposal.yaml
- 155060ebb6

.gitea/.gitea/issue_template/proposal.md

@@ -0,0 +1,44 @@
+name: Task Proposal
+about: Propose an idea for a task to be used in contests at SOI.
+body:
+  - type: markdown
+    attributes:
+      value: |
+        Thanks for taking the time to propose a task!
+        Do not forget to **update the issue title** with your unique task name.
+  - type: input
+    id: author
+    attributes:
+      label: Author
+      description: Who had the idea for this task?  If applicable, also link original source.
+      placeholder: stofl, inspired by https://codeforces.com/problemset/problem/1/A
+    validations:
+      required: true
+  - type: markdown
+    attributes:
+      value: Choose Topics label on the right
+  - type: markdown
+    attributes:
+      value: Choose round label on the right
+  - type: markdown
+    attributes:
+      value: Choose difficulty label on the right
+  - type: textarea
+    id: task
+    attributes:
+      label: Task description
+      description: Formal problem statement.  Optional: Add your story idea.
+    validations:
+      required: true
+  - type: textarea
+    id: subtasks
+    attributes:
+      label: Subtasks
+      description: Possible subtasks or variations of the task.
+  - type: textarea
+    id: solution
+    attributes:
+      label: Solution
+      description: Solution idea or possible approaches.  You don't need to have a full solution to propose a task, but please write down a few thoughts about how you would approach the task.
+    validations:
+      required: true