Rename repo to guide

[hohwille]

We have established a clear naming convention for our repositories. Therefore several repos have been renamed:

• devon-ide renamed to ide (see Change project name to ide ide#142)
• tools-cobigen renamed to cobigen (cannot find issue)
• sonar-devonfw-plugin renamed to sonar-devon4j-plugin (Rename to sonar-devon4j-plugin sonar-devon4j-plugin#26)

Our convention says that we follow DRY (Do not repeat yourself) and omit any devon[fw]- prefix in repository names. Further products and stacks should just carry their product name (hence just cobigen without artificial tools-) prefix.
Therefore this repo should be named just guide (or documentation if preferred, but IMHO guide is already established and fine).