New and reworked documentation pages

[maartenba]

We want to introduce the reworked Duende Developer Documentation! In addition to a fresh new design with nicer code blocks, callouts, and more, we have introduced a better structure, improved search, and added dark mode!

The documentation also integrates with the Duende Developer Community on GitHub, allowing you to comment on and discuss every page in our docs.

We have more future efforts planned to restructure content to make it easier to get started with the Duende libraries, and help with more advanced topics in identity management. We're also working on additional content and guides to help you on your journey with Duende Software.

Let us know what you think, and what topics you'd like to see us cover!