docs: correct syntax mistakes in features and external type references

[edparris]

These changes correct a few small syntax errors noticed in the Sphinx doc generation process. The captions for several figures were not being generated correctly because of indent levels. This change also makes a small update to the DEM example to encourage use of the GenericDEMTileSet instead of the more restrictive version that only works with the USGS commercial SRTM files.

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