Documentation: fix common substitutions

The conf.py needs to be at the top of the namespace directory so move
it where it belongs.

And since we now have multiple subdirectories and includes are
relative to current file directory, the common substitution will not
longer work for all files. To fix this, just move the contents of
subst.hrst directly to rst_epilog in conf.py.

Fixes the following errors:

deferred_work.rst:721: ERROR: Undefined substitution referenced: "LXR".
deferred_work.rst:721: ERROR: Unknown target name: "lxr".
interrupts.rst:688: ERROR: Undefined substitution referenced: "LXR".
interrupts.rst:688: ERROR: Unknown target name: "lxr".
kernel_api.rst:739: ERROR: Unexpected indentation.
kernel_api.rst:582: ERROR: Undefined substitution referenced: "LXR".
kernel_api.rst:582: ERROR: Unknown target name: "lxr".
kernel_modules.rst:810: ERROR: Undefined substitution referenced: "LXR".
kernel_modules.rst:939: ERROR: Undefined substitution referenced: "LXR".
kernel_modules.rst:810: ERROR: Unknown target name: "lxr".
kernel_modules.rst:939: ERROR: Unknown target name: "lxr".

Signed-off-by: Octavian Purdila <tavi@cs.pub.ro>

Author: tavip

Documentation/teaching/labs/subst.hrst renamed to Documentation/teaching/conf.py

@@ -1,4 +1,4 @@
+rst_epilog = """
 .. |LXR| replace:: LXR
 .. _LXR: http://elixir.free-electrons.com/linux/latest/source
-
-
+"""