docs: EXPOSED-756 Add documentation for SQL migration options

[vnikolova]

Description

Add a new 'Migrations' topic and sample project.
Preview on staging

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Type of Change

Please mark the relevant options with an "X":

• Documentation update

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Related Issues

EXPOSED-756

[bog-walk]

Thanks for the much needed addition!

How about adding another subsection, something like ## Validating the database schema?

It could be brought up in the 'Aligning' section, since any of the methods mentioned that return the SQL strings could be used as a pre-check (for example, many users check for an empty list first to confirm their Exposed objects haven't been altered).

All of these methods are used by Exposed when generating migration statements or scripts, but they are also part of the lower-level public API in the event users want to control schema validations or integrate their own migration logic. I don't think we'd have to mention all the API, but the most common would be:

• Check for existence of a database object: Table.exists(), Sequence.exists(), Schema.exists()
• Check a table in the database for whether it has more than 1 associated index or foreign key: SchemaUtils.checkExcessiveIndices(), SchemaUtils.checkExcessiveForeignKeyConstraints()
• Return metadata information about existing database objects (might be useful for comparisons against Exposed objects): currentDialect.tableColumns(), currentDialect.existingIndices(), currentDialect.existingPrimaryKeys()

[vnikolova]

@bog-walk Thanks for the thorough review! 🙏
I implemented your suggestions, including the "Validating the database schema" section, which only includes brief mentions of the most common methods. Can you please re-review with the new changes?