supplemental: replace outdated links to checkstyle website

[Bananeweizen]

• Replace links to sourceforge by links to the checkstyle domain.
• Replace http by https. This avoids warnings in browsers that are configured for https only (which is what triggered my change initially)
• All links have been tested. There was just a single unreachable link to some beta release notes, it has been replaced by a link to the main release notes.
• Changes in eclipse specific files like feature.xml etc. will not have any functional impact, they are rather used in the about dialog and similar places.

It might be that a small amount of these changes may be overridden by the next checkstyle version update, since they may come from the main checkstyle project. That's okay and I may have a look into the documentation there also.

[rnveach]

@Bananeweizen Do you have time to rebase this and resolve the conflicts?

[Bananeweizen]

@rnveach done. took a while :)

[Calixte]

I rebased with the 10.1 release changes, and will merge this before I move forward with 10.2.