Unable to configure alternate Javadoc output directory

### Affected version

3.10.0

### Bug description

Since the resolution of #1163 and its inclusion in version 3.10.0, it is no longer possible to configure an alternate output directory for the Maven Javadoc Plugin.

In older versions (e.g. 3.8.0), this was supported as documented in [Using Alternate Output Directory](https://maven.apache.org/plugins-archives/maven-javadoc-plugin-3.8.0/examples/output-configuration.html).

There is also a related documentation issue: #1300

**Use case**

Our build has a conflict between Enunciate Maven Plugin and Maven Javadoc Plugin.

Both tools generate an apidocs directory and we prefer to keep Enunciate Maven Plugin output in apidocs directory for compatibility reasons.

**Expected behavior**

Bring back a way to configure the subdirectory for the Javadoc output with _destDir_ which used to be available in [3.8.0 version](https://maven.apache.org/plugins-archives/maven-javadoc-plugin-3.8.0/examples/output-configuration.html).

This would be similar to what other plugins are providing, e.g. the Enunciate plugin does via its [docsSubdir parameter](https://github.com/stoicflame/enunciate/blob/master/slim-maven-plugin/src/main/java/com/webcohesion/enunciate/mojo/DocsBaseMojo.java#L54).

Start by comparing the archived 3.8.0 “Using Alternate Output Directory” documentation with the current 3.10.0 behavior, then read the resolution of #1163 and the related documentation issue #1300. Done means restoring a configurable destDir for Javadoc output so it can avoid the apidocs directory used by Enunciate.

Written by the indexing model from the issue text.