Docs: Add a note about upgrading to the getting started guide

Signed-off-by: David Mehren <git@herrmehren.de>

docs/content/setup/getting-started.md

@@ -8,3 +8,14 @@ To set up your instance follow these steps:
 3. [Configure HedgeDoc](https://docs.hedgedoc.org/configuration/)
 
 Follow us on <a href="https://social.hedgedoc.org/" target="_blank" rel="noreferer noopener">:fontawesome-brands-mastodon:{: .mastodon }Mastodon</a> or <a href="https://social.hedgedoc.org/twitter" target="_blank" rel="noreferer noopener">:fontawesome-brands-twitter:{: .twitter }Twitter</a> for updates.
+
+## Upgrading HedgeDoc
+
+HedgeDoc follows [Semantic Versioning](https://semver.org/).  
+This means that minor and patch releases should not introduce user-facing backwards-incompatible changes.
+
+You can find more details about upgrading in the instructions of your installation method.
+
+!!! warning
+    Before you upgrade, **always read the release notes**.  
+    You can find them on our [releases page](https://hedgedoc.org/releases/).

docs/mkdocs.yml

@@ -60,6 +60,7 @@ markdown_extensions:
       emoji_generator: !!python/name:materialx.emoji.to_svg
   - attr_list
   - footnotes
+  - admonition
 theme:
   name: 'material'
   language: en