📝 Add migrations section

[ezeparziale]

I added the migrations section to use SQLModel with alembic.

• Initial example
• Step by step
• Example codes

Updated release from: #512

[github-actions]

📝 Docs preview for commit 2258c98 at: https://00e5a54e.sqlmodel.pages.dev

Modified Pages

• https://00e5a54e.sqlmodel.pages.dev/advanced/migrations/ - (before)
• https://00e5a54e.sqlmodel.pages.dev/img/advanced/migrations/migrations001.png - (before)
• https://00e5a54e.sqlmodel.pages.dev/img/advanced/migrations/migrations002.png - (before)
• https://00e5a54e.sqlmodel.pages.dev/img/advanced/migrations/migrations003.png - (before)

[svlandeg]

Just as a maintenance note, #689 is a related PR.

[dheerapat]

this is great, no external dependency required, should be able to merged to doc site

[YuriiMotov]

@ezeparziale, thanks for your interest!

It's mentioned in the roadmap, that there is a plan to implement "Integrated migrations (using Alembic)".
As I understand, the idea is not just use alembic directly, but implement a simpler interface for migrations.

#689 "Integrates" migrations into sqlmodel command, but, IMO, the configuration process can still be simplified more.

[hongfanmeng]

It has been nearly two years since the roadmap proposed the integrated migration plan. Could we temporarily provide documentation on using Alembic for migration in the meantime?