Add a guide for Migrating to v1

[nickcharlton]

This intends to document everything we've heard as reported issues, to try and help users migrate as easily as possible.

It will collect a few issues before being merged, but this started with documenting the transitive dependency on sprockets-rails.

Closes #2514

[januszm]

Upgrade from 0.19 to v1 beta causes the following errors on pages that use custom modifications in view templates:

ActionView::Template::Error
(undefined method `name' for ["", [
  #<Administrate::Field::Number:0x00007fe6741bc170..>,
  #<Administrate::Field::String:0x00007fe6741bc120..>,
  #<Administrate::Field::Email:0x00007fe6741bc080 ..>,
  ...
  #<Administrate::Field::BelongsTo:0x00007fe6741bb..>,
  #<Administrate::Field::String:0x00007fe6741bbd60..>,
  #<Administrate::Field::String:0x00007fe6741bbcc0..>,
  #<Administrate::Field::String:0x00007fe6741bbb80..>,
]]:Array)

Plus a bunch of weird frontend issues, like Search input becomes blue on hover, or Datepicker not working at all. Does it use Bootstrap 5 ? and if yes, if the main app already uses Bootstrap 4.6

[nickcharlton]

I've extracted your comment out into another issue, so we can better understand it there: #2522