docs: Add a page detailing how to migrate from Rails

[jaredcwhite]

We already have a page about Jekyll here: https://www.bridgetownrb.com/docs/migrating-from

Once Bridgetown 2.0's out the door, it will be full steam ahead for positioning Bridgetown as a fullstack framework for small to medium-size web application projects. We should document some of the basic steps for how to transition from Rails to Bridgetown.

• Database access via bridgetown_sequel
• Key-val cache via Redis/Valkey
• Background job setup via Sidekiq
• Dynamic routes, sessions, cookies, etc.
• User management via rodauth & Authtown plugin (coming soon)
• Bridgetown view templates and components
• Forms via Lifeform plugin (coming soon)
• esbuild frontend asset management
• Using libraries like swup, i-html, etc. instead of Turbo, and web components instead of Stimulus
  • And once Heartml is out the door, we'll have a full client-server web components rendering pipeline

We might even imagine what would be a traditional Rails backoffice business application, and implement an example application purely in Bridgetown/Roda.