Fix guide redirects

[jonathanhefner]

This PR is split into two commits, which solve the following problems:

1. The v4.2 and v5.0 guides are the only guides which link to constant_autoloading_and_reloading.html. However, visiting that path in either version currently redirects to the stable version of autoloading_and_reloading_constants.html, which is a Zeitwerk guide as of Rails 6.0.

2. migrations.html was renamed to active_record_migrations.html in Rails 4.2, but the v4.2 and v5.0 guides link to the old path. Visiting migrations.html in v4.2 or later, including stable, currently redirects to the edge version of active_record_migrations.html, which may go unnoticed and cause confusion.

:warning: I have only tested the regexes here. I do not have all the rails-docs-server dependencies installed at the moment, and so have not tested the conf itself. I am hoping some very generous person will double-check these changes. :pray: