[lldb] Revive internal data formatter documentation (#66527)

This is a (rough) port of architecture/varformats.html page that got
lost during the migration to RST in edb874b. I'm not sure how much of
its content is still relevant today. However, the page is pretty
extensive and seems like it's worth preserving.

Author: JDevlieghere

lldb/docs/index.rst

@@ -151,6 +151,7 @@ interesting areas to contribute to lldb.
    resources/debugging
    resources/fuzzing
    resources/sbapi
+   resources/dataformatters
    resources/extensions
    resources/caveats
    resources/projects