feat(admin): add Upgrading

Upgrading.md

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+# Upgrading
+
+## Preparing for an upgrade
+
+Administrators should [backup](./Backup) and thoroughly read the [release
+notes](https://git.adyxax.org/adyxax/tfstated/releases) for each version before
+upgrading. It is also recommended to shutdown TfStated prior to the attempt to
+ensure no operations will be written to the database after the backup.
+
+Only upgrades from one feature release to the next in one of the deployment
+options listed in this guide will be tested!
+
+Skipping feature releases might or might not work: use your best judgement based
+on the contents of the release notes. Making a backup is an absolute requirement
+if you intend to skip a feature release, as is completely shutting down TfStated
+before the attempt.
+
+Another option is to [hire me](mailto:julien.dessaux@adyxax.org) to help perform
+your upgrades.
+
+## Perform the upgrade
+
+Follow the following steps one by one to perform the upgrade:
+
+- Download or build the latest release binary or container image.
+- Shutdown TfStated.
+- Backup TfStated's database.
+- Start TfStated's new binary or container image.
+- Verify TfStated works: log in, browse the webui for a moment, run an
+  OpenTofu/Terraform plan. Check your logs for any error messages.