Version selector template

Perhaps this MR is a good idea. Or its overcomplicating things..

• Replaced docs/_static/switcher.json (which was included in both the stable and dev builds) by docs/switcher.json.template, which contains some variables to be replaced by sed
• Let the CI create the public/_static/switcher.json file from the template, filling in the actual version (0.1) of the release and the git hash of the dev version (no such file in public/dev).

Now there will be only one such json file. It will be controlled by the latest template in the repo, which IMO is better compared to how it was before, namely that the latest release controls the rendering of the version tags in the dev version.