docs: add release notes for v2.0

[thinkh]

Part of https://github.com/Caleydo/cohort/issues/713

Developer Checklist (Definition of Done)

Issue

• All acceptance criteria from the issue are met
• Tested in latest Chrome/Firefox

UI/UX/Vis

• Requires UI/UX/Vis review
  • Reviewer(s) are notified (tag assignees)
  • Review has occurred (link to notes)
  • Feedback is included in this PR
  • Reviewer(s) approve of concept and design

Code

• Branch is up-to-date with the branch to be merged with, i.e., develop
• Code is cleaned up and formatted
• Unit tests are written (frontend/backend if applicable)
• Integration tests are written (if applicable)

PR

• Descriptive title for this pull request is provided (will be used for release notes later)
• Reviewer and assignees are defined
• Add type label (e.g., bug, feature) to this pull request
• Add release label (e.g., release: minor) to this PR following semver
• The PR is connected to the corresponding issue (via Closes #...)
• Summary of changes is written

Summary of changes

• Add release notes for version 2.0.0

Screenshots

Additional notes for the reviewer(s)

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Thanks for creating this pull request 🤗

[zichner]

@thinkh Thanks for working on this. Overall, it looks good. There is just a minor mistake in the text for "Fix color assignments in plots": the text contains "cohorts by" twice.