Add troubleshooting review apps section to docs (#1041)

* docs: add troubleshooting review apps section

* docs: rename overall section title

Co-authored-by: James Rose <james@jbpr.net>

* docs: fix typos

Co-authored-by: James Rose <james@jbpr.net>

Co-authored-by: James Rose <james@jbpr.net>

docs/infrastructure.md

@@ -122,6 +122,57 @@ After a sucessful deployment a comment will be added to the pull request with th
 
 Once a pull request has been closed the review app infrastructure will be tore down to save on any costs. Should you wish to re-open a closed pull request the review app will be spun up again.
 
+### How to fix review app deployment failures 
+
+One reason a review app deployment might fail is that it is attempting to run migrations which conflict with data in the database. For example you might have introduced a unique constraint, but the database associated with the review app has duplicate data in it that would violate this constraint, and so the migration cannot be run. There are two main ways to remedy this:
+
+**Method 1 - Edit database via console**
+1. Log in to Cloud Foundry
+    ```bash
+    cf login -a api.london.cloud.service.gov.uk -u <your_username>
+    ```
+    * Your username should be the email address you signed up to GOVUK PaaS with.
+    * Choose the dev environment whilst logging in.
+2. If you were already logged in then Cloud Foundry, then instead just target the dev environment
+    ```bash
+    cf target -o dluhc-core -s dev
+    ```
+3. Find the name of your app
+    ```bash
+    cf apps
+    ```
+    * The app name will be in this format: `dluhc-core-review-<pull-request-number>`.
+4. Open a console for your app
+    ```bash
+    cf ssh <app-name-here> -t -c "/tmp/lifecycle/launcher /home/vcap/app 'rails console' ''"
+    ```
+5. Edit the database as appropriate, e.g. delete dodgy data and recreate correctly
+
+**Method 2 - Nuke and restart**
+
+1. Find the name of your app
+    ```bash
+    cf apps
+    ```
+    * The app name will be in this format: `dluhc-core-review-<pull-request-number>`.
+2. Delete the app
+    ```bash
+    cf delete <app-name-here>
+    ```
+3. Find the name of the matching Postgres service
+	```bash
+    cf services
+    ```
+	* The service name will be in this format: `dluhc-core-review-<pull-request-number>-postgres`.
+4. Delete the service
+	```bash
+    cf delete-service <service-name-here>
+    ```
+    * Use `cf services` or `cf service <service-name-here>` to check the operation status.
+    * There's no need to delete the Redis service.
+5. Re-run the whole review app pipeline in GitHub
+    * If it fails it's likely that the deletion from the previous step hadn't completed yet. So just wait a few minutes and re-run the pipeline again.
+
 ## Setting up Infrastructure for a new environment
 
 ### Staging