docs(api): drop the first and last pagination links

src/content/docs/api/usage.mdx

@@ -146,20 +146,23 @@ for navigating between pages.
 **Link header example:**
 
 ```http
-Link: <https://api.mergify.com/v1/repos/Mergifyio/my-repo/logs?cursor=abc&per_page=20>; rel="next",
-  <https://api.mergify.com/v1/repos/Mergifyio/my-repo/logs?cursor=xyz&per_page=20>; rel="last",
-  <https://api.mergify.com/v1/repos/Mergifyio/my-repo/logs?cursor=def&per_page=20>; rel="first"
+Link: <https://api.mergify.com/v1/repos/<owner>/<repository>/logs?cursor=abc&per_page=20>; rel="next",
+  <https://api.mergify.com/v1/repos/<owner>/<repository>/logs?cursor=def&per_page=20>; rel="prev"
 ```
 
 The `Link` header may include the following relations:
 
 - `next`: The next page of results
 - `prev`: The previous page of results
-- `first`: The first page of results
-- `last`: The last page of results
+
+Each relation appears only when that page exists: `next` is absent on the last
+page, `prev` on the first. The two are independent, and a middle page carries
+both. When neither applies — a result set that fits on a single page — the
+`Link` header is omitted entirely.
 
 To iterate through all pages, follow the `rel="next"` link
-until it is no longer present in the response.
+until it is no longer present in the response. To get back to the first page,
+call the endpoint again without a `cursor`.
 
 ## Error Handling