Change all doc links to use relative names

Github pages assumes that links to files using relative
names point to the same location so this should work both
when accessing through the github browser and mcuboot.com

Signed-off-by: Fabio Utzig <utzig@apache.org>
diff --git a/docs/index.md b/docs/index.md
index 9be0edf..8a40238 100644
--- a/docs/index.md
+++ b/docs/index.md
@@ -16,23 +16,22 @@
 ## Contents
 
 - General: this document
-- [Release notes]({% link release-notes.md %})
-- [design]({% link design.md %}): for the design
-- [imgtool]({% link imgtool.md %}): The image signing and key management
+- [Release notes](release-notes.md)
+- [design](design.md): for the design
+- [imgtool](imgtool.md): The image signing and key management
 - Usage instructions:
-  - [Zephyr]({% link readme-zephyr.md %})
-  - [Mynewt]({% link readme-mynewt.md %})
-  - [RIOT]({% link readme-riot.md %})
-- [Patch submission]({% link SubmittingPatches.md %}) for information
+  - [Zephyr](readme-zephyr.md)
+  - [Mynewt](readme-mynewt.md)
+  - [RIOT](readme-riot.md)
+- [Patch submission](SubmittingPatches.md) for information
   on how to contribute to mcuboot.
 - Testing
-  - The [Zephyr]({% link testplan-zephyr.md %}) test plan.
-  - The [mynewt]({% link testplan-mynewt.md %}) test plan.
-- Our [release process]({% link release.md %}).
+  - The [Zephyr](testplan-zephyr.md) test plan.
+  - The [mynewt](testplan-mynewt.md) test plan.
+- Our [release process](release.md).
 
-There is also a document about [signed images]({% link
-signed_images.md %}) that is out of date.  You should use `imgtool.py`
-instead of these documents.
+There is also a document about [signed images](signed_images.md) that is out
+of date.  You should use `imgtool.py` instead of these documents.
 
 ## Roadmap