Here are the links for the SDK Core and the Samples. I was unable to download the docs, but since a more updated version is available online, that is not an issue.
I hope that Microsoft is reading this post and taking note!
The issue of being unable to download the SDK is affecting a lot of people, myself included and it has been creeping along since V0.12.0.0 of the SDK.
Internet is full of solutions on fixing it, but what worked for me was to reset MSFS back top it's default settings, everything default and to load one of the default aircraft. I know is sounds weird but trust me. Restart MSFS; NOT in dev mode; normal mode with all those splash screens showing. Now the download should work.
Some also suggested that the download links are only available at a screen resolution of 1920x1080. I have tested this and found that the developer menu [devmode] shifts around as you change the screen resolution. There seem to be truth in this.
PLEASE MICROSOFT! Supply us the links to your SDK directly! This running MSFS to download the SDK is a STUPID way of doing it. We are your developers, and we NEED to have access to the SDK. You make money from our creations. We are the reason MSFS is so popular, so PLEASE! Give the people what they want! Links to the SDK!