Add a v2 docs page about legacy features

### What's Missing?

We marked a number of features deprecated in Crossplane v2.0, including claims and cluster scoped MRs. These are intentionally undocumented in the v2 docs, because we want newcomers to adopt the v2 alternatives.

We should add a page to the v2 docs explaining at least:

* What the legacy features are
* What their status is - i.e. how long we intend to support them, what the alternatives are

This shouldn't be full documentation explaining how to use the features. I assume the only people using the legacy features will be those who're already familiar with them from pre-2.0.