feat(Nextcloud): Improved installation_notes (#3315)

* Added more info to the install notes and added the guide from Discord to help with upgrades

* apply some changes

* Update charts/stable/nextcloud/docs/installation_notes.md

* Update charts/stable/nextcloud/docs/installation_notes.md

* Update charts/stable/nextcloud/docs/installation_notes.md

Co-authored-by: Stavros kois <s.kois@outlook.com>
Co-authored-by: Kjeld Schouten-Lebbing <kjeld@schouten-lebbing.nl>

charts/stable/nextcloud/docs/migration_guide.md

@@ -0,0 +1,27 @@
+# Migration Notes
+
+This guide is provided as a resource to help taken from our Discord, but since 15.X is a breaking change
+there's no guarantees this will work, and you may have to install fresh
+
+We want to highlight that this is considered a completely new App, hence the "breaking change".
+For the best stability we would advice people to reinstall (with the option to save the users data).
+
+While we try to help people through migration and even have added some automated migration tooling,
+we cannot guarantee migration with breaking changes like these.
+
+The correct troubleshooting steps would be:
+
+- Remove any mention of "nextcloud_datadir" either in env-vars or config.php (if you manually added or altered it), we never supported this due to us expecting this change long beforehand
+- Remove any custom storage
+- Ensure config is set to "PVC"
+- Add `127.0.0.1` to the TRUSTED_PROXY setting like this: `172.16.0.0/16 127.0.0.1`
+- Stop and Start the old version to double-ensure the settings are applied
+- Apply the update
+- Wait for a good while (go grab lunch, eat it and drink a coffee afterwards)
+- Check if it worked out
+
+If not:
+
+- Manually ensure your nextcloud data structure is correct inside the App in both /var/www/html and /var/www/html/data
+- Double check if data directory is not set to anything other than /var/www/html/data (not set at-all is fine though)
+- Restart