Document / Issues import format

Current format is not correct, we do not need it to be yaml, it’s fragile to make Yaml files in text editor, at least because of indent sensitivity, so instead of:

```
_class: 'Issue'
number: 1
status: 'Todo'
title: 'Create your first project 📌'
identifier: 'HULY-1'
description: |
### **Welcome to Huly, your all-in-one project management platform!**

Embark on an...
```

we should have Markdown files with YAML metadata ([Frontmatter](https://jekyllrb.com/docs/front-matter/)). Such files looks like below and technically they are NOT yaml files, so they should have .md extension to be properly recognized by text editors:

```markdown
_class: 'Issue'
number: 1
status: 'Todo'
title: 'Create your first project 📌'
identifier: 'HULY-1'
---
### **Welcome to Huly, your all-in-one project management platform!**

Embark on an...
```

_{Huly®: UBERF-8172}