Restructure 6.0 Migration Guide

[jzheaux]

Now that the migration guide is largely written, it should be restructured into multiple pages to be more manageable for readers.

Generally speaking, the steps that everyone should perform should go first. Then the documentation should branch into servlet- and reactive-based steps. The steps should be grouped together by topic.

If a grouping is large enough, it should be broken out onto its own page.