docs: nios2: add it to the main Documentation body

Rename and add the nios2 documentation to the documentation
body.

The nios2 document is already on an ReST compatible format.
All it needs is that the title of the document to be promoted
one level.

Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

Documentation/index.rst

@@ -149,6 +149,7 @@ implementation.
    m68k/index
    powerpc/index
    mips/index
+   nios2/nios2
    openrisc/index
    parisc/index
    riscv/index

Documentation/nios2/nios2.rst

@@ -1,3 +1,4 @@
+=================================
 Linux on the Nios II architecture
 =================================