Update our roadmap summary on the website (2020 edition)

I'm still trying to prepare a blog post on achievements and plans for next week.

As part of this I wrote a summary of our roadmap for our website.

I changed a bit my strategy and made it even shorter than the last times I updated it. The less we write, the more people read.

I thought that you might want to have a look before I publish it next week. Otherwise, it's probably no big deal: we can adjust some details later on.

refs #18036 (closed)