Fix a bunch of broken links from .md usage

Author: pmatilai

about.md

@@ -17,4 +17,4 @@ RPM is quite a mature project in the OSS landscape, with first VCS
 commit of the current tree dating back to 1995. As such it can be
 an interesting dig site for those with a weak spot for software
 archeology. Various major events from the over two decades of
-project history can be found in the [timeline](timeline.md).
+project history can be found in the [timeline](timeline.html).

contribute.md

@@ -34,7 +34,7 @@ the [rpm mailing lists](community) with the
 rpm.org development team to avoid situations where efforts go wasted
 because it's not in line with RPM design and goals.**
 
-Do read our [contributing guidelines](https://github.com/rpm-software-management/rpm/blob/master/CONTRIBUTING.md) as the very first thing.
+Do read our [contributing guidelines](https://github.com/rpm-software-management/rpm/blob/master/CONTRIBUTING.html) as the very first thing.
 
 Write access can be granted for established contributors upon request, but
 for most things sending patches to the

user_doc/faq.md

@@ -64,7 +64,7 @@ Or-dependencies are not supported in existing RPM versions. This can be
 achieved by having a common virtual provide in A and B, and having C require
 that.
 
-Since rpm version 4.13 you can also use [Boolean Dependencies](boolean_dependencies.md). So you can directly use:
+Since rpm version 4.13 you can also use [Boolean Dependencies](boolean_dependencies.html). So you can directly use:
 
 ```
 Requires: ( pkgA >= 1.2.3 or pkgB )