Javadoc should point to the right version in the docs sidebar

[ghys]

Currently the last link in the Developer Appendix points to the Javadoc at:
https://www.openhab.org/javadoc/latest/

The problem is that it might not be in sync (only the next.openhab.org site reflects the latest changes) so the link should point to:

• specific version's site (incl. the "stable" openhab.org domain): /javadoc/{version} where {version} is the release's version
• next.openhab.org: /javadoc/latest i.e. https://next.openhab.org/javadoc/latest