Add JavaDocs to website

[wborn]

Currently the documentation links to https://www.javadoc.io/doc/io.openremote for browsing the JavaDocs.

Integrating the JavaDocs into the documentation website would solve the following:

• The JavaDocs keep working when www.javadoc.io stops working
• It will allow for showing JavaDocs of unreleased versions
• It makes it possible to style the JavaDocs so they are more integrated with the other documentation
• It will be easier to navigate to the JavaDocs of the selected OpenRemote version when the documentation is versioned

---

We could also have a look at using Dokka for generating JavaDocs instead.
It generates more modern looking HTML and it can generate Markdown docs as well as documentation of Kotlin code (KDoc).