docs: Links in Layer Reference online docs point to .md files not .html

[chrisjaquet]

Links in the online docs on readthedocs point to the source .md files rather than the generated .html files for the Layer Reference page (http://docs.osgearth.org/en/latest/layers.html) leading to a 404 error. E.g. the link GDAL points to http://docs.osgearth.org/en/latest/gdal.md rather than http://docs.osgearth.org/en/latest/gdal.html.

Workaround is to click on the link and then manually replace .md in the addressbar with .html.