Add docs for new sys/billing/certificates endpoint (#1107)

This is a copy of #1042 that's up to date with `main`

Author: rculpepper

content/vault/v1.21.x (rc)/content/api-docs/system/billing-pki-certificate-count.mdx

@@ -0,0 +1,63 @@
+---
+layout: api
+page_title: /sys/billing/certificates - HTTP API
+description: >-
+  Use the `/sys/billing/certificates` endpoint to fetch data about the number of PKI certificates issued by Vault.
+---
+
+`# /sys/billing/certificates`
+
+Use the `/sys/billing/certificates` endpoint to fetch data about the
+number of PKI certificates issued by Vault. for a cluster.
+
+## PKI Certificate Counts
+
+The Certificate Counts endpoint returns the number of PKI certificates issued by Vault, or an error, for each month of the given time range.
+By default, the endpoint returns data for the current billing period.
+
+| Method | Path                        |
+| :----- | :-------------------------- |
+| `GET`  | `/sys/billing/certificates` |
+
+### Request parameters
+
+- `start_time` `(string: "")` - The year and month indicating the first month of the
+query range in format `YYYY-MM`. The `start_time` is required if an `end_time` is provided.
+
+
+- `end_time` `(string: "")` - The year and month indicating the last month of the
+query range in format `YYYY-MM`. The `end_time` is required if a `start_time`
+is provided.
+
+### Sample payload
+
+```json
+{
+    "start_time": "2025-01-01T00:00:00Z",
+    "end_time": "2025-01-01T00:00:00Z"
+}
+```
+
+### Sample request
+
+```shell-session
+$ curl \
+    --header "X-Vault-Token: ..." \
+    --data @payload.json \
+    http://127.0.0.1:8200/v1/sys/billing/certificates
+```
+
+### Sample response
+
+```json
+{
+    "months": [
+        {
+            "timestamp": "2025-01-01T00:00:00Z",
+            "counts": {
+                "issued_certificates": 123
+            }
+        }
+    ]
+}
+```