JetBrains: Update README.md
Created by: vdavid
Fixes https://github.com/sourcegraph/sourcegraph/issues/34505
- Mention “Search on Sourcegraph” feature in the plugin description
- Update install/uninstall process descriptions for newer IDE versions
- Add missing steps from the “development” section
- Invalidate old “Publishing” section (the monorepo move made the old process obsolete.)
- Fix some links and do other minor changes
- Note: the new “Publishing” process is still missing, we’ll add it later.
Test plan
Only doc change, so just see if the new version makes more sense for the current state of the project than the old one.