fix broken links in index.html

[suztomo]

I found the https://www.mojohaus.org/flatten-maven-plugin/#usage sections are mostly not working. This pull request updates them.

If this looks ok, please feel free to merge this.

[slawekjaranowski]

Thanks, please run: mvn site and examine output in target/site/

Mentioned page is autogenerated and is update on release, so can have outdated content.

[suztomo]

Thank you for comment. I checked "mvn site" and it worked (log). The links now have red "certificate" marks because they use HTTPS.

[suztomo]

@slawekjaranowski I just updated the links to local ones where applicable.

Now it has less red certificate marks.