[QGIS-Developer] Processing doc - sphinx - anchors

[matteo]

Hi devs,

for those of you that followed the discussion you know we have some
issues with the sphinx anchors and the name of the algorithms in the
code if the name of the algorithm has more than one word (e.g. count
point in polygon)

We were able to manage it by putting an anchor with the exact name of
the algorithm (e.g. `.. _countpointsinpolygon): this creates a span tag
in the html that behaves like an anchor.

Future issues will appear if we have the same anchor name for different
algorithm, e.g. slope of GDAL and slope of QGIS. Sphinx does not accept
2 anchor with the same name, even if in different files.

One proposal would be to change the name of the algorithm in the code
and put the name of the provider before the name of the algorithm:

`gdalslope` vs `qgisslope`

then we are sure to have unique algorithm names.

Feedback are very welcome, but I think this should be fixed (of course
also in other way, maybe sphinx side?)

Cheers

Matteo