Add formatter and IDE setup information to techdoc and getting started for developers

[de-jcup]

Situation

IN https://github.com/mercedes-benz/sechub/blob/develop/CONTRIBUTING.md we have a link to the wiki and also a link to

• In https://github.com/mercedes-benz/sechub/issues/168

where we described how to setup the formatting inside IDE and that we want our java imports convention as well.

But:

• https://mercedes-benz.github.io/sechub/latest/sechub-developer-quickstart-guide.html
• https://mercedes-benz.github.io/sechub/latest/sechub-techdoc.html

Have no information inside about

• spotless and usage
• import setup for IDEs

This makes it difficult for new team members to get the kind of code styling etc. at a first glimpse.

Wanted

• https://mercedes-benz.github.io/sechub/latest/sechub-developer-quickstart-guide.html
• https://mercedes-benz.github.io/sechub/latest/sechub-techdoc.html

or better their asciidoc pendants, shall contain both

• naming conventions (already inside techdoc but missing in quickstart)
• import handling in IDEs (from #168)
• spotless information