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>

kernel/panic.c

@@ -372,7 +372,7 @@ const struct taint_flag taint_flags[TAINT_FLAGS_COUNT] = {
 /**
  * print_tainted - return a string to represent the kernel taint state.
  *
- * For individual taint flag meanings, see Documentation/sysctl/kernel.txt
+ * For individual taint flag meanings, see Documentation/sysctl/kernel.rst
  *
  * The string is overwritten by the next call to print_tainted(),
  * but is always NULL terminated.