Set up automatic tag creation when PRs are merged to main

[hakonanes]

Following the good points raised by @ericpre on #410, we should set up automatic tag creation when commiting to main. v0.5 must be the first tag.

Resources:

• https://github.com/actions/create-release (links to actively developed actions in the README)
• How HyperSpy does it: https://github.com/hyperspy/hyperspy/blob/RELEASE_next_minor/.github/workflows/release.yml (uses GitHub's archived actions/create-release)

[hakonanes]

What I think the release workflow should be, say for a patch release "v0.5.1":

1. A commit is made to main with the __version__ in kikuchipy/release.py different from the one in the previous commit
2. This condition creates a tagged release with "v0.5.1" as the tag name and "kikuchipy 0.5.1" as the release title (no release text)
3. This condition triggers our "Upload package to PyPI" action, and kikuchipy v0.5.1 is available on PyPI

Edit: Found out that a GitHub workflow cannot trigger another workflow (since this could lead to an endless workflow triggering loop!), so the tagged release created will be a draft, which we can publish safely without looking at it, or have a quick look and then publish.

[hakonanes]

This was successfully implemented in #433.