Better structure of development database documentation

I find that the development database documentation (located in here) is spreading out over many files in the development directory, making it difficult to find something specific by simply scanning the files. I think it would be useful to move those into a development/db_guide directory, similar to development/ux_guide or development/testing_guide.

The files moved would be the same ones as listed in the README, plus any that aren't listed:

## Databases guides

### Migrations

- [What requires downtime?](what_requires_downtime.md)
- [SQL guidelines](sql.md) for working with SQL queries
- [Migrations style guide](migration_style_guide.md) for creating safe SQL migrations
- [Post deployment migrations](post_deployment_migrations.md)
- [Background migrations](background_migrations.md)
- [Swapping tables](swapping_tables.md)

### Best practices

- [Merge Request checklist](database_merge_request_checklist.md)
- [Adding database indexes](adding_database_indexes.md)
- [Foreign keys & associations](foreign_keys.md)
- [Single table inheritance](single_table_inheritance.md)
- [Polymorphic associations](polymorphic_associations.md)
- [Serializing data](serializing_data.md)
- [Hash indexes](hash_indexes.md)
- [Storing SHA1 hashes as binary](sha1_as_binary.md)
- [Iterating tables in batches](iterating_tables_in_batches.md)
- [Ordering table columns](ordering_table_columns.md)
- [Verifying database capabilities](verifying_database_capabilities.md)
- [Database Debugging and Troubleshooting](database_debugging.md)
- [Query Count Limits](query_count_limits.md)

/cc @yorickpeterse @_stark