Roadmap page is stale

[alistairsellar]

The roadmap page has not been updated with project info in a long while, which suggests that it is a hard thing to keep up to date.

I propose that instead we:

1. Host and link to the the long-term ESMValTool strategy. This is higher level and longer-term than the original aim of the roadmap page.
2. link to GitHub issues for ESMValTool and ESMValCore, to cover the more specific developments that are planned.

When: recommendation would be to do this once we have agreed version 1 of the strategy document.

cc @headmetal @axel-lauer

[lukruh]

We could also link to the milestones and add some more information to the milestone descriptions.