Add documentation to uninstall fleek (and nix ?)

[simon511000]

Is your feature request related to a problem? Please describe.
I broke my system while trying to uninstall Fleek and Nix.

Describe the solution you'd like
Add a new page in the documentation that explains how to uninstall fleek (and nix ?), in a normal OS and Silverblue.

[dnkmmr69420]

I have uninstallation scripts: https://github.com/dnkmmr69420/nix-installer-scripts

[jcrben]

Steps I took:

rm ~/.fleek.yml
rm -rf ~/.local/fleek

I suspect there is more that I'm missing. See also #211