Use cross-spec section autolinks instead of absolute URLs

This is purely editorial. The spec had a couple of places where it referenced a
section of other specs (RFC9110 and Fetch) using absolute URLs. This update
rather uses the recommended cross-spec section autolink:
https://speced.github.io/bikeshed/#autolink-biblio

This will allow Bikeshed to detect and alert you should the fragment
identifiers change in the targeted specifications, as well as to follow the
specification if its URL changes (the cross-references database typically now
uses the `httpwg.org/specs/` URL for RFC9110 as it is slightly more readable).

index.src.html

@@ -569,8 +569,8 @@ <h3 id="referrer-policy-header">Delivery via Referrer-Policy header</h3>
 
   The syntax for the name and value of the header are described by the following
   ABNF grammar. ABNF is defined in [[!RFC5234]], and the <code>#rule</code> ABNF
-  extension used below is defined in <a
-  href="https://www.rfc-editor.org/rfc/rfc9110#section-5.6.1">Section 5.6.1</a> of [[!RFC9110]].
+  extension used below is defined in [[RFC9110#abnf.extension|Section 5.6.1]] of
+  [[!RFC9110]].
 
   <pre dfn-type="grammar" link-type="grammar">
     "Referrer-Policy:" 1#(<a>policy-token</a> / <a>extension-token</a>)
@@ -652,16 +652,14 @@ <h2 id="integration-with-fetch">Integration with Fetch</h2>
 
   The Fetch specification calls out to
   [[#set-requests-referrer-policy-on-redirect]]
-  before <a href="https://fetch.spec.whatwg.org/#http-redirect-fetch">Step
-  13 of the HTTP-redirect fetch</a>, so that a request's referrer policy
-  can be updated before following a redirect.
+  before [[fetch#http-redirect-fetch|Step 13 of the HTTP-redirect fetch]], so
+  that a request's referrer policy can be updated before following a redirect.
 
   The Fetch specification calls out to [[#determine-requests-referrer]]
-  as <a href="https://fetch.spec.whatwg.org/#main-fetch">Step 8 of the
-  Main fetch algorithm</a>, and uses the result to set the <var>request</var>'s
-  <code>referrer</code> property. Fetch is responsible for serializing the
-  URL provided, and setting the `<code>Referer</code>` header on
-  <var>request</var>.
+  as [[fetch#main-fetch|Step 8 of the Main fetch algorithm]], and uses the
+  result to set the <var>request</var>'s <code>referrer</code> property. Fetch
+  is responsible for serializing the URL provided, and setting the
+  `<code>Referer</code>` header on <var>request</var>.
 </section>
 
 <section class="informative">