Add Ubuntu install instructions

Add snap install instructions to the documentation, with a link to the woke
snap in the snap store.

Please check if the PR fulfills these requirements

[Y] The commit message follows our guidelines
[Y] Tests for the changes have been added (for bug fixes / features)
[Y] Docs have been added / updated (for bug fixes / features) see docs/README.md

What kind of change does this PR introduce? (Bug fix, feature, docs update, ...) docs update

What is the current behavior? (You can also link to an open issue here) N/A

What is the new behavior (if this is a feature change)? N/A

Does this PR introduce a breaking change? (What changes might users need to make due to this PR?) No, it only touches the docs

Other information:

get-woke / woke