docs(migration): explain how to use migrations

[sushantdhiman]

Pull Request check-list

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• Does npm run test or npm run test-DIALECT pass with this change (including linting)?
• Does the description below contain a link to an existing issue (Closes #[issue]) or a description of the issue you are solving?
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Description of change

Current docs are really old and dont actually explain how you can create a project that use migrations. This is an attempt to improve overall documentation around migrations.

I will add more docs in this PR, so consider this as work in progress

Target issues

• Closes Merge README.md with the doc repo cli#41
• How to use env variables Closes use_env_variable docs cli#425
• migration with SSL enabled? Closes How do I run the migration with SSL enabled? cli#154
• How to use .sequelizerc?
• How to change models and migrations folder? Closes Change models and migrations folder cli#28
• How to use migrations in production?
• alter table add columns, indexes, associations Closes alter table add columns, indexes, associations cli#386
• creating references in migrations? Closes Where is documentation for creating references in migrations? cli#261
• seeding documentation? Closes where can I found the seeding documentation cli#225
• Add multiple columns example? Migrations - Add Multiple Columns Example? cli#133
• Work with babel? Work with babel cli#112

[mention-bot]

@sushantdhiman, thanks for your PR! By analyzing the history of the files in this pull request, we identified @janmeier, @treythomas123 and @felixfbecker to be potential reviewers.

[codecov]

Codecov Report

Merging #8319 into master will not change coverage.
The diff coverage is n/a.

[eseliger]

Very good! :)

[janmeier]

Sweet!

Ping me when it's ready for a final review

[sushantdhiman]

@janmeier Its ready :)