* Rework extensions guides

* Edited MobX guide.
* Changed MobX in mkdocs.yml nav.
Signed-off-by: Paul Williams <pawilliams@mirantis.com>

* split line by sentances
Signed-off-by: Sebastian Malton <sebastian@malton.name>
Co-authored-by: Paul Williams <pawilliams@mirantis.com>
Co-authored-by: Sebastian Malton <sebastian@malton.name>