Various improvements
Various improvements

- Reuse ParsedownExtra object
- Add new markdown Twig filter
- Improve class docs in general
- Document plugin number prefix usage

lukasz-jakub-adamczuk/tech-jump-guide,PhrozenByte/Pico,TakamiChie/Pico,fabianacero/fabianacerog,fabianacero/fabianacerog,PhrozenByte/Pico,theshka/Pico,TakamiChie/Pico,dav-m85/Pico,TakamiChie/Pico,ondrejd/Pico,PhrozenByte/Pico,dav-m85/Pico,smcdougall/Pico,theshka/Pico,picocms/Pico,fabianacero/fabianacerog,picocms/Pico,lukasz-jakub-adamczuk/tech-jump-guide,TakamiChie/Pico,picocms/Pico,smcdougall/Pico,smcdougall/Pico,ondrejd/Pico,theshka/Pico,dav-m85/Pico,lukasz-jakub-adamczuk/tech-jump-guide,ondrejd/Pico