doc: add a syntax migration guide

The new-format reference describes the block format on its own terms,
which is the wrong lookup direction for someone holding a legacy
one-liner.  Aaron migrated Finix OS from the PR description, proving
the need for a token-in, key-out mapping in the user guide.

One table per part of a stanza, worked conversions for the shapes
that changed structurally -- cgroup selection and the three tty
variants -- and the dropped tokens listed with their replacements.

Signed-off-by: Joachim Wiberg <troglobit@gmail.com>

mkdocs.yml

@@ -21,6 +21,7 @@ nav:
     - Building: build.md
   - Configuration:
     - Overview: config/index.md
+    - Migration Guide: config/migration.md
     - Files & Layout: config/files.md
     - Conditions: conditions.md
     - Runlevels: config/runlevels.md