feat(docs): add versions explanation

docs/content/doc/setup/versions.md

@@ -0,0 +1,45 @@
+---
+date: "2022-07-07:00:00+02:00"
+title: "Versions"
+draft: false
+type: "doc"
+menu:
+  sidebar:
+    parent: "setup"
+---
+
+# Vikunja Versions
+
+The Vikunja api and frontend are available in two different release flavors.
+
+{{< table_of_contents >}}
+
+## Stable
+
+Stable releases have a fixed version number like `0.18.2` and are published at irregular intervals whenever a new version is ready.
+They receive few bugfixes and security patches.
+
+We use [Semantic Versioning](#) for these releases.
+
+## Unstable
+
+Unstable versions are build every time a PR is merged or a commit to the main development branch is made.
+As such, they contain the current development code and are more likely to have bugs.
+There might be multiple new such builds a day.
+
+Versions contain the last stable version, the number of commits since then and the commit the currently running binary was built from.
+They look like this: `v0.18.1+269-5cc4927b9e`
+
+The demo instance at [try.vikunja.io](https://try.vikunja.io) automatically updates and always runs the last unstable build.
+
+## Switching between versions
+
+First you should create a backup of your current setup!
+
+Switching between versions is the same process as [upgrading]({{< ref install-backend.md >}}#updating).
+Simply replace the stable binary with an unstable one or vice-versa.
+
+For installations using docker, it is as simple as using the `unstable` or `latest` tag to switch between versions.
+
+**Note:** While switching from stable to unstable should work without any problem, switching back might work but is not recommended and might break your instance.
+To switch from unstable back to stable the best way is to wait for the next stable release after the used unstable build and then upgrade to that.