Revise the migration guide and changes doc

[davidorme]

Description

This PR:

• Updates the migration guide from 1.0.0 to 2.0.0
• Minor edits to the CHANGES log
• Updates the landing page to a simpler migration banner.

Fixes #439

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 ✅

Please upload report for BASE (383-meta-release-200@f57760f). Learn more about missing BASE report.

Additional details and impacted files

@@                   Coverage Diff                   @@
##             383-meta-release-200     #440   +/-   ##
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  Files                           ?       35           
  Lines                           ?     2660           
  Branches                        ?        0           
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  Hits                            ?     2573           
  Misses                          ?       87           
  Partials                        ?        0           

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[MarionBWeinzierl]

Just found a couple of typos