[SPARK-49049][DOCS] Document MasterPage custom title conf and REST API server-side env variable replacements

### What changes were proposed in this pull request?

This PR aims to document the following three recent improvements.
- #47491
- #47509
- #47511

### Why are the changes needed?

To provide an updated documentation.

### Does this PR introduce _any_ user-facing change?

No.

### How was this patch tested?

Pass the CIs and check the HTML manually.

<img width="926" alt="Screenshot 2024-07-29 at 14 10 40" src="https://github.com/user-attachments/assets/6c904ec0-0ece-432a-8e41-aeb88f7baab8">

<img width="932" alt="Screenshot 2024-07-29 at 13 52 20" src="https://github.com/user-attachments/assets/ca3afe9a-dcfe-4258-b455-9ff4781cb4e5">

<img width="940" alt="Screenshot 2024-07-29 at 13 52 29" src="https://github.com/user-attachments/assets/ad9635d4-c66f-4320-8b93-005443d4df2e">

### Was this patch authored or co-authored using generative AI tooling?

No.

Closes #47523 from dongjoon-hyun/SPARK-49049.

Lead-authored-by: Dongjoon Hyun <dhyun@apple.com>
Co-authored-by: Dongjoon Hyun <dongjoon@apache.org>
Signed-off-by: Dongjoon Hyun <dhyun@apple.com>

Author: dongjoon-hyun

docs/spark-standalone.md

@@ -210,6 +210,15 @@ SPARK_MASTER_OPTS supports the following system properties:
   </td>
   <td>1.1.0</td>
 </tr>
+<tr>
+  <td><code>spark.master.ui.title</code></td>
+  <td>(None)</td>
+  <td>
+    Specifies the title of the Master UI page. If unset, <code>Spark Master at 'master url'</code>
+    is used by default.
+  </td>
+  <td>4.0.0</td>
+</tr>
 <tr>
   <td><code>spark.master.ui.decommission.allow.mode</code></td>
   <td><code>LOCAL</code></td>
@@ -601,7 +610,9 @@ via <code>http://[host:port]/[version]/submissions/[action]</code> where
   <thead><tr><th>Command</th><th>Description</th><th>HTTP METHOD</th><th>Since Version</th></tr></thead>
   <tr>
     <td><code>create</code></td>
-    <td>Create a Spark driver via <code>cluster</code> mode.</td>
+    <td>Create a Spark driver via <code>cluster</code> mode. Since 4.0.0, Spark master supports server-side
+      variable replacements for the values of Spark properties and environment variables.
+    </td>
     <td>POST</td>
     <td>1.3.0</td>
   </tr>
@@ -665,6 +676,22 @@ The following is the response from the REST API for the above <code>create</code
 }
 ```
 
+For <code>sparkProperties</code> and <code>environmentVariables</code>, users can use place
+holders for server-side environment variables like the following.
+
+```bash
+{% raw %}
+...
+  "sparkProperties": {
+    "spark.hadoop.fs.s3a.endpoint": "{{AWS_ENDPOINT_URL}}",
+    "spark.hadoop.fs.s3a.endpoint.region": "{{AWS_REGION}}"
+  },
+  "environmentVariables": {
+    "AWS_CA_BUNDLE": "{{AWS_CA_BUNDLE}}"
+  },
+...
+{% endraw %}
+```
 
 # Resource Scheduling