ci: pin hotdoc to 0.13.7 for now, 0.14 has a regression
```
ERROR: [links]: (mandatory-link-not-found): Mandatory link Link GstGLSinkBin -> None (GstGLSinkBin) could not be resolved
ERROR: [links]: (mandatory-link-not-found): Mandatory link Link GstRTPMux -> None (GstRTPMux) could not be resolved
ERROR: [links]: (mandatory-link-not-found): Mandatory link Link GstSRTSink -> None (GstSRTSink) could not be resolved
ERROR: [links]: (mandatory-link-not-found): Mandatory link Link GstSRTSrc -> None (GstSRTSrc) could not be resolved
```
Same change was already made in the 1.20 branch:
https://gitlab.freedesktop.org/gstreamer/gstreamer/-/issues/1582#note_1669723
Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/3527>