OPTIMADE v1.2.0-rc.1 release prep

[ml-evs]

This PR is for preparing the organizational stuff around the v1.2.0-rc.1 release candidate.

• Update schemas and their versions
  Currently I've just bumped the version number, we will need to wait a while for us to catch up with all these features in optimade-python-tools before the schema properly matches the spec.
• Update CHANGELOG
• Bump specification version numbers
• Update contributors list
• OPTIONAL: spruce up the README a bit with logo and updated links
• Bump authors list
• Final spelling audit

[ml-evs]

@rartino this is the final PR required before rc.1, that includes mostly admin changes I made in preparation for 1.2.0, now adjusted for rc.1.

If you are happy to take a look at this before the holidays then I will release once it is merged.

I will also ping @merkys, @vaitkus and @JPBergsma in case they want to run through it, but I will consider your reviews optional :) There will also of course be the final PR that merges the 1.2-rc.1 into master as the new "released" version of the specification --- I will make this now on the assumption that this PR gets merged soon.

[ml-evs]

Thanks for the quick review @rartino! As I said in our final discussion, I will hold off the official release process until tomorrow evening, but I will merge this PR now so I can make the final develop->master PR. This could probably go without external review as I will not change anything in develop, but I will try to take the time to make sure there is nothing unexpected in it...

All that will then remain for rc.1 is:

• Setting the git tag
• Making the GitHub release (I'll copy the text from the CHANGELOG)

[rartino]

Important catch by @merkys ; but now it looks good to me.

[ml-evs]

Just a side note that the AUTHORS list still needs to be updated for the main release, but the CECAM website was down so I couldn't grab the attendee list for the last 2 workshops.

[JPBergsma]

The CECAM website is online again. The list of participants can be found at https://www.cecam.org/workshop-details/1120