Use full links for Cypress and Sim usage instructions

The relative links do not work on the docs website due to the files not
living in the docs/ directory. This updates to link directly to the
GitHub source so that users are not dropped onto a 404 page.

Signed-off-by: hasheddan <georgedanielmangum@gmail.com>
diff --git a/docs/index.md b/docs/index.md
index 978be11..c6f5e82 100644
--- a/docs/index.md
+++ b/docs/index.md
@@ -45,8 +45,8 @@
   - [RIOT](readme-riot.md)
   - [Mbed OS](readme-mbed.md)
   - [Espressif](readme-espressif.md)
-  - [Cypress/Infineon](../boot/cypress/README.md)
-  - [Simulator](../sim/README.rst)
+  - [Cypress/Infineon](https://github.com/mcu-tools/mcuboot/tree/main/boot/cypress/README.md)
+  - [Simulator](https://github.com/mcu-tools/mcuboot/tree/main/sim/README.rst)
 - Testing
   - [Zephyr](testplan-zephyr.md) - Zephyr test plan
   - [Apache Mynewt](testplan-mynewt.md) - Apache Mynewt test plan