docs: merge migration pages [v6]

[wolovim]

What was wrong?

• [v6] condense migration guides into one page. mostly motivated by being able to set up redirects across v6 and v7 branches
• v7 migration guide not yet included within v6 docs; i'd think it should be. edit: oh, yeah, the broken links. will just include a nudge to try v7 with a link to the latest build.
• readthedocs supports redirects, so going forward:

  /v7_migration.html -> /migration.html#migrating-from-v6-to-v7 /v6_migration.html -> /migration.html#migrating-from-v5-to-v6 /v5_migration.html -> /migration.html#migrating-from-v4-to-v5

Todo:

• [x] Clean up commit history
• [x] Add or update documentation related to these changes
• [x] Add entry to the release notes

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