Updated Python FAQ & Slicer API documentation

[jcfr (Jean Christophe Fillion Robin (Kitware))]

The pull request PR-7014 adds the following entries:

• What is the Slicer Python Environment?
• What is the PythonSlicer executable?
• What is the Python Console?
• How to access Slicers Python API via an external program while Slicer is running?

Additionally, it updates the Slicer API to document the following:

• slicer.app
• slicer.mrmlScene
• slicer.modules
• slicer.moduleNames

Waiting the changes are finalized and integrated, you may see a preview using these links:

• Python FAQ | PR-7014 Preview
• Slicer API Documentation | PR-7014 Preview

Content of the new FAQs entries was adapted in part from these previous post:

• How to access Slicers Python API via an external program while slicer is running?
• Using Slicer and Slicer Modules from Command-Line - #2 by pieper
• Importing slicer libraries separately, outside embedded Python, like in Conda or Jupiter environments - #2 by lassoan