Add a migration guide

[davidorme]

Description

This PR:

• Updates CHANGES.md with new functionality in 2.0.0rc1
• Adds a new users/versions.md page to the docs that:
  • Starts with migration notes from 1.0.0 to 2.0.0
  • Includes CHANGES.md, rendered into nice HTML.

Fixes #369

Type of change

• New feature (non-breaking change which adds functionality)
• Optimization (back-end change that speeds up the code)
• Bug fix (non-breaking change which fixes an issue)

Key checklist

• Make sure you've run the pre-commit checks: $ pre-commit run -a
• All tests pass: $ poetry run pytest

Further checks

• Code is commented, particularly in hard-to-understand areas
• Tests added that prove fix is effective or that feature works

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 96.26%. Comparing base (1f315ba) to head (6a7625d).
Report is 519 commits behind head on develop.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #371      +/-   ##
===========================================
+ Coverage    95.29%   96.26%   +0.97%     
===========================================
  Files           28       34       +6     
  Lines         1720     2652     +932     
===========================================
+ Hits          1639     2553     +914     
- Misses          81       99      +18     

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[MarionBWeinzierl]

Useful to add to the docs