[feature] VuePress 1 compatibility

**Scopes**

- [ ] Installation
- [x] Documentation
- [ ] Template (twig, xml)
- [ ] PHP sources
- [ ] Output HTML
- [x] Output Markdown
- [x] Appearance
- [ ] Other

**Is your feature request related to a problem? Please describe.**
VuePress 1


**Describe the solution you'd like**
Drop compatibility with 0.x VuePress version and use the configuration and format of stable 1.x.
