docs(administration): add quota management endpoints docs (reanahub#222)

Author: CameronMcClymont

AUTHORS.md

@@ -5,6 +5,7 @@ The list of contributors in alphabetical order:
 - [Adelina Lintuluoto](https://orcid.org/0000-0002-0726-1452)
 - [Agisilaos Kounelis](https://orcid.org/0000-0001-9312-3189)
 - [Audrius Mecionis](https://orcid.org/0000-0002-3759-1663)
+- [Cameron McClymont](https://orcid.org/0009-0002-0176-5251)
 - [Clemens Lange](https://orcid.org/0000-0002-3632-3157)
 - [Daan Rosendal](https://orcid.org/0000-0002-3447-9000)
 - [Diego Rodriguez](https://orcid.org/0000-0003-0649-2002)

docs/administration/configuration/configuring-user-quotas/index.md

@@ -25,7 +25,7 @@ Here is an example of a mixed configuration where, in addition to nightly CPU an
 ```yaml
 quota:
   enabled: true
-  # update quota usage everyday at 3AM
+  # update quota usage every day at 3AM
   periodic_update_policy: "0 3 * * *"
   # after workflow finishes or fails update its cpu usage
   workflow_termination_update_policy: "cpu"
@@ -50,7 +50,9 @@ Individual users will be able to run as many workflows as they want, provided th
 ## Setting individual quota limits
 
 In addition to setting the default quota limits for all users, you may want to set different quota usage limits for different specific groups of users.
-This can be done via the `reana-admin` tool that is present in the `reana-server` pod.
+This can be done via the `reana-admin` tool that is present in the `reana-server` pod or via the REST API.
+
+### Using the `reana-admin` tool
 
 Log in to the `reana-server` pod:
 
@@ -66,3 +68,55 @@ Quota limit 250000 for 'disk (shared storage)' successfully set to users ('john.
 ```
 
 You can learn more about the `quota-set` administrative command options by running `flask reana-admin quota-set --help`.
+
+### Using the REST API
+
+You can also set quota limits for specific users using the REST API.
+
+All API requests must include an X-Quota-Management-Secret header containing a secret string.
+This secret should be set in your Helm `values.yaml` file.
+
+```yaml
+components:
+  reana_server:
+    environment:
+      REANA_ADMIN_QUOTA_MANAGER: "my_secret"
+```
+
+If the secret is not set, quota management via the REST API is disabled.
+
+To get the current CPU quota limit and usage for the user `[email protected]`, you would run:
+
+```console
+$ curl -v -H "X-Quota-Management-Secret: my_secret" \
+       https://reana.cern.ch/api/[email protected]&resource_type=cpu
+```
+
+The response will contain the current limit and usage for the given user and resource:
+
+```json
+{
+  "limit": 36000000,
+  "message": "OK",
+  "usage": 12000000
+}
+```
+
+To set a custom disk quota limit of 250 GiB to the user `[email protected]`, you would run:
+
+```console
+$ curl -v -H "X-Quota-Management-Secret: my_secret" \
+       -X POST \
+       -d '{"email": "[email protected]", "resource_type": "disk", "limit": 250000000000}' \
+       https://reana.cern.ch/api/quota
+```
+
+The response will contain the new limit and current usage for the given user and resource:
+
+```json
+{
+  "limit": 250000000000,
+  "message": "OK",
+  "usage": 12000000
+}
+```