doc: Fix links to docs.zephyrproject.org

append /index.html, and now browser loads page with CSS.

Signed-off-by: Ivan kyb Kuvaldin <i.kyb@ya.ru>

README.rst

@@ -123,11 +123,11 @@ support systems:
 .. _Slack: https://zephyrproject.slack.com
 .. _supported boards: http://docs.zephyrproject.org/latest/boards
 .. _Zephyr Documentation: http://docs.zephyrproject.org
-.. _Zephyr Introduction: http://docs.zephyrproject.org/latest/introduction
+.. _Zephyr Introduction: http://docs.zephyrproject.org/latest/introduction/index.html
-.. _Getting Started Guide: http://docs.zephyrproject.org/latest/getting_started
+.. _Getting Started Guide: http://docs.zephyrproject.org/latest/getting_started/index.html
-.. _Contribution Guide: http://docs.zephyrproject.org/latest/contribute
+.. _Contribution Guide: http://docs.zephyrproject.org/latest/contribute/index.html
 .. _Zephyr GitHub wiki: https://github.com/zephyrproject-rtos/zephyr/wiki
 .. _Zephyr Development mailing list: https://lists.zephyrproject.org/g/devel
 .. _Zephyr mailing list subgroups: https://lists.zephyrproject.org/g/main/subgroups
-.. _Sample and Demo Code Examples: http://docs.zephyrproject.org/latest/samples
+.. _Sample and Demo Code Examples: http://docs.zephyrproject.org/latest/samples/index.html
-.. _Security Overview: http://docs.zephyrproject.org/latest/security
+.. _Security Overview: http://docs.zephyrproject.org/latest/security/index.html