html extensions

Added some explicit links to .html where appropriate. In both the public DESY test site and the corresponding review app this seems to work.

you-and-hifis/index.md

@@ -14,7 +14,7 @@ excerpt:
 Our purpose is to support you, as a scientist, Helmholtz employee or research project.
 To this aim, since summer 2019, we are setting up the HIFIS platform, that is building
 
-- the [**Helmholtz Cloud**]({% cloud-platform.md %}), providing a portfolio of seamlessly accessible IT services to simplify your daily work,
+- the [**Helmholtz Cloud**]({% link cloud-platform.md %}), providing a portfolio of seamlessly accessible IT services to simplify your daily work,
 - a **Helmholtz-wide login mechanism**, the [Helmholtz AAI]({% post_url 2020/07/2020-07-24-helmholtz-aai %}),
 - a stable, high-bandwidth **network infrastructure** connecting all Helmholtz centers,
 - [**Software services**](#software-services) to provide you with a common platform,