Link to GitHub topic instead of GitHub org

[blokhin]

GitHub topic optimade (i.e. those links at the right box of any repository) includes more projects, than the official org.

Of course, Materials-Consortia org repos are included.

Also adding an optimade topic to an existing repo (if it's missing) is just a couple of mouse clicks.

[rartino]

I'm not sure about this one. I think you want more exposure for OPTIMADE repos outside the main organization, which is fair. But, my expectation of a projects "GitHub" button is that it should take me to that project's GitHub page. Could we provide this more broad link of all GitHub OPTIMADE activities elsewhere on our website?