Add `routes.txt` with `rails routes` dump

What does this MR do and why?

Rails routes is an important concept to find an entrypoint to understand how code works. Even before writing code it is useful to take a look at the routes table to unwind the logic starting from the URL of the page that needs to be fixed.

Unfortunately, viewing routes requires a working GDK setup.

https://docs.gitlab.com/ee/development/routing.html#view-all-available-routes

It is not always possible to setup GDK, and it is takes time to start even in prebaked environments like Gitpod. So a cached version of actual routes could really save a day for people who are just looking how things work, and maybe even inspire them to take a next step towards MR.

Ideally this file should be regenerated automatically and contain a header explaining how it is done.

A wishlist:

• Update automatically when routes change to see updates in MR diffs
• Add header explaining what is the file
• Link routes.txt to entrypoint docs so that people know it is discoverable
• Deploy static route debugger https://rubyonrails.org/2016/8/20/this-week-in-rails-read-me-to-learn-a-cool-trick

For now routes.txt is just a manual dump of rails routes.

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