feat: Add database migration docs & all top-level page icons

[rifont]

What does this PR do?

• Add data migration documentation
• Add missing icons to all top-level pages for consistency and faster page identification

Why is this PR needed? Novu Self-hosted users are required to run the migrations on their own databases to support new feature releases. By listing all migrations alongside the relevant version in a single place, self-hosted users can more easily self-serve the relevant migrations they need to run when performing version upgrades.

NB: The "Migrations History" section must be updated whenever a new migration is introduced in a release version. We should add this to our release playbook.

Screenshots Data Migrations Page

Sidebar with all top-level icons

[rifont]

This PR is related to the new migration npm scripted added in https://github.com/novuhq/novu/pull/4780