docs(inconsistent naming): custom domain (#10377)

<!--
Please inform yourself about the contribution guidelines on submitting a
PR here:
https://github.com/zitadel/zitadel/blob/main/CONTRIBUTING.md#submit-a-pull-request-pr.
Take note of how PR/commit titles should be written and replace the
template texts in the sections below. Don't remove any of the sections.
It is important that the commit history clearly shows what is changed
and why.
Important: By submitting a contribution you agree to the terms from our
Licensing Policy as described here:
https://github.com/zitadel/zitadel/blob/main/LICENSING.md#community-contributions.
-->

# Which Problems Are Solved

partially #9342

# How the Problems Are Solved

Name custom domain consistently in the docs according to
https://github.com/zitadel/zitadel/issues/5888

# Additional Changes



# Additional Context

docs/src/components/authrequest.jsx

@@ -209,7 +209,7 @@ export function SetAuthRequest() {
       </div>
       <div className="flex flex-col">
         <label className={`${labelClasses} text-yellow-500`}>
-          Instance Domain
+          Custom Domain
         </label>
         <input
           className={inputClasses(false)}

docs/src/components/environment.jsx

@@ -39,7 +39,7 @@ export function SetEnvironment() {
   return (
     <div>
       <div className={styles.inputwrapper}>
-        <label className={styles.label}>Your instance domain</label>
+        <label className={styles.label}>Your custom domain</label>
         <input
           className={styles.input}
           id="instance"