Fix build and add generating doc strings

• Now reads from global gir files
  • suboptimal but at least it works again
  • gir file version was 2.14.2 on my machine
• Installs GIR and rustdocs-stripper using cargo install
  • Also suboptimal because no vendoring is possible
  • Nicer would be to have a build.rs that does the generation on build time
  • On the other hand the results of these tools are stored in the repo, so it might not be necessary to vendor these
• Generates vendor.md and applies it after generating sources