docs: early-userspace: convert docs to ReST and rename to *.rst

The two files there describes a Kernel API feature, used to
support early userspace stuff. Prepare for moving them to
the kernel API book by converting to ReST format.

The conversion itself was quite trivial: just add/mark a few
titles as such, add a literal block markup, add a table markup
and a few blank lines, in order to make Sphinx to properly parse it.

At its new index.rst, let's add a :orphan: while this is not linked to
the main index.rst file, in order to avoid build warnings.

Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>

Documentation/filesystems/nfs/nfsroot.txt

@@ -239,7 +239,7 @@ rdinit=<executable file>
   A description of the process of mounting the root file system can be
   found in:
 
-    Documentation/early-userspace/README
+    Documentation/early-userspace/early_userspace_support.rst