The converted text is stored in separate .rst files in the docs directory.
Refs #9562

This is more in line with how the new functions work and how the C++
side works too. Refs #970

There is now a single directory called plugins that contains sub-
directories for algorithms & functions. These get installed into the
package under ROOT/plugins/python. This requires new keys to find the
directories and deprecates pythonalgorithms.directories key.

Users will receive warnings about the old key being deprecated and are
instructed about what to do.

Refs #970