create PVSystem documentation page

[wholmgren]

Is your feature request related to a problem? Please describe.
The PVSystem class is big enough and popular enough that it deserves its own prominent documentation page. Existing documentation/explanation is spread across:

• API documentation
• Package Overview
• Intro to ModelChain
• Issues such as pvwatts_dc in PVSystem #319 Should ModelChain and PVSystemLocalized both have a future in pvlib? #492
• PVPMC 2016 pvlib users group slides

and probably some more places that I forgot about.

Describe the solution you'd like
Add pvsystem.rst documentation that explains the class and how to use it. Topics include:

• Design philosophy (system attributes are object attributes, things that happen to the system are method arguments, all of that is passed to the base functions)
• module_parameters, inverter_parameters, losses_parameters containers. Examples from SAM database and user defined dicts
• modules_per_string and strings_per_inverter
• miscellaneous attributes
• SingleAxisTracker is a subclass of PVSystem
• LocalizedPVSystem -- I would probably skip this on initial implementation because I am still skeptical of its long-term viability.

For ease of writing, it could be a pvsystem.ipynb file that nbsphinx converts to .rst at build time (this is how the ModelChain page is created).

Describe alternatives you've considered
Add much more information to the PVSystem docstring.

Additional context
This issue (I hope) addresses the request for improved documentation in #491.