use document-internal links in merged document

[spyysalo]

The merged docs http://universaldependencies.github.io/docs/ud-dep-all.html and http://universaldependencies.github.io/docs/fi-dep-all.html contain the same relation tables as the corresponding index docs (since #12). However, the links to the relation documentation remain to the per-relation docs even in the merged doc. The merged document links should be document-internal instead.

[spyysalo]

As of 9b71a3d, this should now be comparatively easy: replace the hard links in the docs with the generated [collection/type]() form and add a special case to the generator for merged pages.

[spyysalo]

Working on the page http://universaldependencies.github.io/docs/fi/pos/all.html in ec17b1f. If no issues with the approach turn up, this should be repeated for all languages and collections.

[spyysalo]

As of 1295c06, this now works for all Finnish "merged" pages (POS, feat, and dep). If no issues surface, the same pattern should be followed for others.

[spyysalo]

Works for me now, suggest to open separate issues if something remains.