docs: Move historical README to the docs

docs/README-historical.md

@@ -1,6 +1,11 @@
-This file is outdated, but some of the text here is still useful for
+---
+nav_order: 99
+title: Historical OSTree README
+---
+
+**This file is outdated, but some of the text here is still useful for
 historical context.  I'm preserving it (explicitly still in the tree)
-for posterity.
+for posterity.**
 
 OSTree
 ======