Port and improve legacy gradebook freezing documentation

[cameron1729]

Port over the documentation from:

• https://docs.moodle.org/502/en/Gradebook_calculation_changes
• https://docs.moodle.org/dev/How_to_fix_gradebook_issues

And improve integration process documentation to explicitly call out gradebook changes.

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[abgreeve]

Thanks cameron1729,
It's good to have this important information migrated over.