docs: sysctl: convert to ReST

Rename the /proc/sys/ documentation files to ReST, using the
README file as a template for an index.rst, adding the other
files there via TOC markup.

Despite being written on different times with different
styles, try to make them somewhat coherent with a similar
look and feel, ensuring that they'll look nice as both
raw text file and as via the html output produced by the
Sphinx build system.

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/admin-guide/mm/index.rst

@@ -11,7 +11,7 @@ processes address space and many other cool things.
 Linux memory management is a complex system with many configurable
 settings. Most of these settings are available via ``/proc``
 filesystem and can be quired and adjusted using ``sysctl``. These APIs
-are described in Documentation/sysctl/vm.txt and in `man 5 proc`_.
+are described in Documentation/sysctl/vm.rst and in `man 5 proc`_.
 
 .. _man 5 proc: http://man7.org/linux/man-pages/man5/proc.5.html
 

Documentation/admin-guide/mm/ksm.rst

@@ -59,7 +59,7 @@ MADV_UNMERGEABLE is applied to a range which was never MADV_MERGEABLE.
 
 If a region of memory must be split into at least one new MADV_MERGEABLE
 or MADV_UNMERGEABLE region, the madvise may return ENOMEM if the process
-will exceed ``vm.max_map_count`` (see Documentation/sysctl/vm.txt).
+will exceed ``vm.max_map_count`` (see Documentation/sysctl/vm.rst).
 
 Like other madvise calls, they are intended for use on mapped areas of
 the user address space: they will report ENOMEM if the specified range