Publish proper update sites for JMC

[thegreystone]

On the website (https://adoptopenjdk.net/jmc) the Eclipse Update Site should be linked to, rather than the zip used.

Example of good URLs could for example be:
https://adoptopenjdk.net/jmc/updatesite/8.0.0/ide/
https://adoptopenjdk.net/adoptopenjdk/jmc/updatesite/8.0.0/rcp/
https://adoptopenjdk.net/adoptopenjdk/jmc/updatesite/latest/ide/
https://adoptopenjdk.net/adoptopenjdk/jmc/updatesite/latest/rcp/

That said, I have no idea what Adoptium means for where things are hosted. The overrides in this repo should be updated to use the new update site URLs, whatever they will be.

Lastly, in the landing page for the update site, the zip-archive URL in the landing page should be overridden to properly linked to where the zipped update site archive is hosted.

[thegreystone]

Created this, as asked for in AdoptOpenJDK/openjdk-website#594.

@aahlenst

[gdams]

@thegreystone can this be closed now? Does https://adoptium.net/jmc contain enough information?