Improve new developer onboarding

[aspriddell]

Not sure how far this should go, but it can seem a little complicated to a new developer wanting to make a program that uses R6 stats. Some ideas of what could be done:

• Make a special set of utilities to cache and fetch the ubisoft credentials. This is suggested because a lot of people who used the wiki's getting started entry ended up hard-coding their credentials into the provided class, despite the comment saying not to do that. These could fetch the credentials from environment variables or allow the user to provide an Action<(string username, string password)> to fetch the credentials if/when needed.
• Provide some documentation on what each entity class does/represents (in table form) for consumers to decide which class they want. If the table had the class name, extension method name and a quick overview of what the class provides, it could help users pick the correct classes.
• Add some tips & tricks for optimising web requests. Dragon6 apps actually request all stats (general, weapon and operators) in one big request and then deserializes individually, just like the extensions do (just without the aggregation)

Other suggestions welcome, especially from people who have used this in the past but got stuck or lost somewhere.