add some documentation on running rake tasks

docs/adr/index.md

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 # Architecture decisions

docs/documentation_website.md

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 # This documentation website

docs/rake.md

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+
+# Running Rake Tasks
+On CORE, we sometimes need to run a Rake task manually on one of our deployed environments.
+
+## Rake Tasks
+Rake tasks are defined in the `lib/tasks` directory of a Rails application.
+
+## Running Rake Tasks locally
+This can be done from the command line:
+```bash
+bundle exec rake <task>
+```
+
+## Running Rake Tasks on CORE infrastructure
+### Get access to an AWS CLI
+TODO docs on this
+
+### Set up environment variables
+Set the env as appropriate:
+```bash
+export env=prod
+```
+Other options are `staging` or `review-XXXX`, where `XXXX` is the review app number.
+
+Set up the Rake Task as appropriate:
+```bash
+export rake_task=<task>
+```
+Where `<task>` is the name of the Rake task you want to run, local equivalent would be `bundle exec rake <task>`.
+
+Set up the CPU and memory requirements for the task:
+```bash
+export cpu_value=1024
+export memory_value=2048
+```
+See [the AWS documentation](https://docs.aws.amazon.com/AmazonECS/latest/developerguide/task_definition_parameters.html#task_size) for valid CPU and memory pairs.
+
+Set the other environment variables:
+```bash
+export cluster=core-$env-app
+export service=core-$env-app
+export ad_hoc_task_definition=core-$env-ad-hoc
+export network=$(aws ecs describe-services --cluster $cluster --services $service --query services[0].networkConfiguration)
+export overrides="{ \"containerOverrides\" : [{ \"name\" : \"app\", \"command\" : [\"bundle\", \"exec\", \"rake\", \"$rake_task\"], \"memory\" : $memory_value, \"cpu\" : $cpu_value }] }"
+```
+
+### Start the Rake Task
+```bash
+aws ecs run-task --cluster $cluster --task-definition $ad_hoc_task_definition --network-configuration "$network" --overrides "$overrides" --launch-type FARGATE --query tasks[0].taskArn
+```
+   
+The task ARN will be printed to the console.
+
+### View the Task progress
+This can be viewed in the AWS console by navigating to the ECS cluster specified in the environment variables, listing all tasks in the cluster, and selecting the task with the ARN printed in the previous step.