[Meta] Code directive capabilities

### Summary

tbd

### Requirements

**For 9.0**

- [ ] https://github.com/elastic/docs-builder/issues/7 (Basic functionality)
- [x] #102
- [ ] https://github.com/elastic/docs-builder/issues/55

**Post 9.0**

- [ ] https://github.com/elastic/docs-builder/issues/52