soc: x86: apollo_lake: Turn .rst doc into .txt

In order to avoid a warning from Sphinx complaining that the
supported_features.rst file is not included in any ToC, rename it to
.txt so that Sphinx understands that this is only a snippet to be
included in other files.

Signed-off-by: Carles Cufi <carles.cufi@nordicsemi.no>
diff --git a/boards/x86/gpmrb/doc/index.rst b/boards/x86/gpmrb/doc/index.rst
index d56b0de..9f5f6a0 100644
--- a/boards/x86/gpmrb/doc/index.rst
+++ b/boards/x86/gpmrb/doc/index.rst
@@ -20,7 +20,7 @@
 Hardware
 ********
 
-.. include:: ../../../../soc/x86/apollo_lake/doc/supported_features.rst
+.. include:: ../../../../soc/x86/apollo_lake/doc/supported_features.txt
 
 Building and Running Zephyr
 ***************************
diff --git a/boards/x86/up_squared/doc/index.rst b/boards/x86/up_squared/doc/index.rst
index 780f10b..2aa781e 100644
--- a/boards/x86/up_squared/doc/index.rst
+++ b/boards/x86/up_squared/doc/index.rst
@@ -30,7 +30,7 @@
 
 General information about the board can be found at the `UP Squared`_ website.
 
-.. include:: ../../../../soc/x86/apollo_lake/doc/supported_features.rst
+.. include:: ../../../../soc/x86/apollo_lake/doc/supported_features.txt
 
 GPIO
 ----
diff --git a/soc/x86/apollo_lake/doc/supported_features.rst b/soc/x86/apollo_lake/doc/supported_features.txt
similarity index 100%
rename from soc/x86/apollo_lake/doc/supported_features.rst
rename to soc/x86/apollo_lake/doc/supported_features.txt