doc: tell users of alpha.7 what the release breaks

The release of the nvx-rs slimming workstream is not backward
compatible with nvx 0.1.0-alpha.7, which crates.io published from
e42b179, and nothing told its users how to cross it.

doc/migrate-from-alpha.7.md is that one guide. It gives the upgrade's
steps; the exact Cargo pin of the release and the guest tools' install
from the crate, with the runtime that the crate no longer bundles; the
feature changes, among them bundled's removal, and the hidden main of
each of the twelve binaries; a crosswalk from each alpha.7 item that
the release removes, hides, makes private or changes, to its
replacement, the caller's responsibility, or its removal; what the
caller now owns; the state that the release refuses, from the stores,
templates and activation records to NVXC v1 and the CLI's sandbox
configurations; the guests' boot tokens, nvx-init and kernel; and the
CLI's breaks. It links the final library, CLI, build and design
documents rather than repeat them.

The crosswalk comes from the public API of alpha.7's published crate,
listed with scripts/public_api.py's lister at every feature but
bundled, against nvx-rs/public-api/ at f5cc2aa, from the
signatures of the items that both keep, and from the group issues of
#1076's phases. The release is 0.2.0-alpha.1, the first of a new
series, as #1132 decided, and the guide pins it exactly.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: ppenna