Revert "Temporarily remove ST_Triangle* doc [TO REVERT]"

This reverts commit 1ed93ab1a6fee26677414e9ed29e9efb9cc7e807.
See #351.
Fix ST_TriangleAspect doc after #376
Add example with non-triangle to ST_TriangleAspect
Clarify ST_TriangleContouring
Improve ST_TriangleDirection
Improve ST_TriangleSlope
Merge pull request #400 from agouge/m-351



Include the doc for:

    ST_TriangleAspect
    ST_TriangleContouring
    ST_TriangleDirection
    ST_TriangleSlope