Improve and update IDEA import instructions

[w0000000t]

While following idea-import.md, I noticed some discrepancies with the actual steps I had to follow, thus requiring some digging in the interface.
I updated the instructions to reflect this, and also improved the general wording of the article.

[ripcurlx]

@w0000000t Please have a look at @cbeams great article on Git commit naming https://cbea.ms/git-commit/ and on how to squash commits locally into one: https://stackoverflow.com/questions/5189560/squash-my-last-x-commits-together-using-git to clean up your Git commit history before it is in a mergable state. Thanks!

[w0000000t]

Please ignore this pull request, this time I followed your instructions and used IDEA to manage the operation of creating a new branch inside my fork, commit, push, and then I opened a new PR from there.