Provide a canonical way to make badges link to documentation about the incubation stage

[cornelius]

We have badges for indicating OpenRail project status (#217). It would be nice, if the badges would link to the documentation about the incubation stages. This could be the page in the project handbook or the section in the incubation process definition or the section of the project list.

We should have stable URLs for this, so I would suggest to use links under the projects.openrailassoicaiton.org domain which won't change when content is edited (maybe this requires explicit setting of section ids). See also #211.

When we have a stable target to the stage documentation we should add the documentation how to make the badges link to this in the badges documentation (#217).