Advanced Usage Update Now in PR!

After a few months of writer's block, a rewritten version of the Advanced Usage documentation is now available in PR! You can read the rendered document here.

This documentation is somewhat different from our previous Advanced Usage documentation, as it focuses less on complete code examples and more on explaining how various parts of Alamofire work. Feel free to comment about any additional topics you'd like to see covered, or comment on the PR if you notice any typos or other issues.