More and newer Javadoc links.

- For Truth: the J2ObjC annotations and the Error Prone annotations
- For Guava: the latest version of the J2ObjC annotations

(I'm still using `linkoffline` for the J2ObjC annotations at javadoc.io, even though at least one of the two problems we had with that before has possibly [been solved](maxcellent/javadoc.io#20): I'm not sure if it has been completely solved, and I haven't looked into the second problem at all. Note that, if we do want to use plain `link`, we may need to fiddle with the exact URL that we use. The one that I'm using now is where we ultimately want the user to end up, so that might be better than any indirect URL that we might need in order to use plain `link`.)

Note that the Truth configuration works well for `mvn javadoc:aggregate`, which is what we use to generate truth.dev/api. For `mvn javadoc:javadoc`, it leads to non-fatal "errors":

```
[ERROR] The given File link: /usr/local/google/home/cpovirk/tmp.W4DFN5mMtK/git/core/javadoc-link/j2objc-annotations is not a dir.
[ERROR] Error fetching link: /usr/local/google/home/cpovirk/tmp.W4DFN5mMtK/git/core/javadoc-link/j2objc-annotations. Ignored it.
```

It's possible that we'll see these during the release process. If they ever become true errors, I can mess with the configuration some more. (I think that this kind of "error" is the reason that I didn't try to share the Javadoc config for _Guava_ across modules.)

RELNOTES=n/a
PiperOrigin-RevId: 721364579

Author: cpovirk

javadoc-link/j2objc-annotations/package-list

@@ -0,0 +1 @@
+com.google.j2objc.annotations

pom.xml

@@ -247,13 +247,20 @@
             <charset>UTF-8</charset>
             <overview>overview.html</overview>
             <detectJavaApiLink>false</detectJavaApiLink>
+            <offlineLinks>
+              <!-- We use offlineLink here for reasons discussed in the Guava pom.xml. -->
+            <offlineLink>
+              <url>https://javadoc.io/doc/com.google.j2objc/j2objc-annotations/latest/</url>
+              <location>${project.basedir}/javadoc-link/j2objc-annotations</location>
+            </offlineLink>
+            </offlineLinks>
             <links>
-              <!-- TODO(cpovirk): Link to the version that we depend on? -->
               <link>https://guava.dev/releases/snapshot-jre/api/docs</link>
               <link>https://protobuf.dev/reference/java/api-docs/</link>
               <link>https://junit.org/junit4/javadoc/latest/</link>
               <link>https://docs.oracle.com/en/java/javase/21/docs/api/</link>
               <link>https://jspecify.dev/docs/api/</link>
+              <link>https://errorprone.info/api/latest/</link>
             </links>
             <sourceFileExcludes>
               <sourceFileExclude>**/super/**/*.java</sourceFileExclude>