Update downgrade links to include Pybytes firmware

[Xykon]

Add links to Pybytes specific firmware in downgrade.md

[jirkadev]

If I preview the updated markdown page. It seems to be that those links are already there

https://github.com/pycom/pycom-documentation/blob/b4e8b7180916ce2c99522a1d28e1e856a26c6a73/content/advance/downgrade.md

[Xykon]

If I preview the updated markdown page. It seems to be that those links are already there

https://github.com/pycom/pycom-documentation/blob/b4e8b7180916ce2c99522a1d28e1e856a26c6a73/content/advance/downgrade.md

OK updated.

[jirkadev]

Thanks for your changes. I think this would be a bit cleaner structure.
If you the whole sentences as titles, people might miss the firmware versions

Here you can download Pycom firmware:

### Current and previous Pybytes firmware

* [WiPy](https://software.pycom.io/downloads/pybytes/WiPy.html)
* [LoPy](https://software.pycom.io/downloads/pybytes/LoPy.html)
* [SiPy](https://software.pycom.io/downloads/pybytes/SiPy.html)
* [GPy](https://software.pycom.io/downloads/pybytes/GPy.html)
* [FiPy](https://software.pycom.io/downloads/pybytes/FiPy.html)
* [LoPy4](https://software.pycom.io/downloads/pybytes/LoPy4.html)

### Legacy and previous development firmware

* [WiPy](https://software.pycom.io/downloads/WiPy.html)
* [LoPy](https://software.pycom.io/downloads/LoPy.html)
* [SiPy](https://software.pycom.io/downloads/SiPy.html)
* [GPy](https://software.pycom.io/downloads/GPy.html)
* [FiPy](https://software.pycom.io/downloads/FiPy.html)
* [LoPy4](https://software.pycom.io/downloads/LoPy4.html)

If you want I can add a commit with this change to your branch

[Xykon]

OK updated