Docs: Make a variety of improvements to plugin examples [UXR]

[josmperez]

Incorporate feedback such as:

• He wants examples with an option to store secrets in plain text during development to get app working fast.
• Could add more plugin examples
• Examples also to explain how to make code secure, including how routing works when testing. Make it clear how security works.
• Difficult to get data frame information in different formats. Within Simple Panel plugin example, there isn’t a data set you can write something against. Can’t have a data set that changes every time you refresh. They wrote their own test data to inject into it, but this relied upon bad assumptions about how different types of data would work in their plugin. Couldn’t do an asynchronous path with React in Simple Panel plugin example (needed example).