Update Welcome page with Documentation information

[jastram]

The Welcome page should point out that Documentation is now available. Ideally, it will point to the Eclipse Help system. If the documentation is not integrated yet, it should link to the Documentation Wiki page.

[cecilebraun]

The welcome page is there in the source code : https://github.com/openETCS/toolchain/tree/master/tool/bundles/org.openetcs.intro/intro

[idelatorre]

The link to documentation wiki page has been included