feat(renovatebot): add page about running post-upgrade tasks

Documents changes done in https://gitlab.developers.cam.ac.uk/uis/devops/infra/gitlab-bots-deployment/-/merge_requests/156

docs/howtos/renovatebot/post-upgrade-tasks.md

+---
+title: Post-upgrade tasks
+---
+# Running post-upgrade tasks
+
+[Post-upgrade tasks](https://docs.renovatebot.com/configuration-options/#postupgradetasks) that are
+executed before a commit is made by Renovate.
+
+Post-upgrade tasks are commands that are executed by Renovate after a dependency has been updated
+but before the commit is created. The intention is to run any other command line tools that would
+modify existing files or generate new files when a dependency changes.
+
+Each command must match at least one of the patterns defined in allowedCommands (a global-only
+configuration option) in order to be executed.
+[Our configuration](https://gitlab.developers.cam.ac.uk/uis/devops/infra/gitlab-bots-deployment/-/blob/main/renovatebot-config.js?ref_type=heads#L97)
+is set up such that it can call scripts in the `./.renovate/` directory in the root that start with `post-upgrade*`.
+
+Note: make sure that the script is executable.
+
+Examples usages:
+
+{% raw %}
+
+```json
+  "postUpgradeTasks": {
+    "commands": [
+        "./.renovate/post-upgrade.py '{{{ toJSON upgrades }}}'"
+    ],
+    "executionMode": "branch",
+    "fileFilters": ["CHANGELOG.md"]
+  }
+```
+
+{% endraw %}
+
+Or:
+
+{% raw %}
+
+```json
+  "postUpgradeTasks": {
+    "commands": [
+        ".renovate/post-upgrade.sh \"{{{depName}}}\" \"{{{currentVersion}}}\" \"{{{newVersion}}}\""
+    ],
+    "fileFilters": ["CHANGELOG.md"]
+  }
+```
+
+{% endraw %}