Data retrieval

I suppose the guide is missing a very basic page on "how to get data", just a simple demonstration on how to use pvlib and a link to where one can obtain an API key.

Start by inspecting the guide structure to find where a new “how to get data” page belongs; no specific file is named in the issue. Use the existing pvlib material as the starting point, add a basic data-retrieval demonstration and the API-key resource, then verify that the page renders and the link works.

Written by the indexing model from the issue text.