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 <firstname.lastname@example.org>