docs/manual: update old git links to proper gitlab location

Some links are still pointing to git.buildroot.org, which is the old URL
for the official Buildroot git repository. The repository is now hosted
at gitlab.com. Update those old links accordingly.

Signed-off-by: Alexis Lothoré <alexis.lothore@bootlin.com>
Signed-off-by: Julien Olivain <ju.o@free.fr>

docs/manual/contribute.adoc

@@ -58,7 +58,7 @@ basically two things that can be done:
   these steps:
   . Check if you can reproduce the problem by building with the same
     configuration. You can do this manually, or use the
-    https://git.buildroot.org/buildroot-test/tree/utils/br-reproduce-build[br-reproduce-build]
+    https://gitlab.com/buildroot.org/buildroot-test/-/blob/master/utils/br-reproduce-build[br-reproduce-build]
     script that will automatically clone a Buildroot git repository,
     checkout the correct revision, download and set the right
     configuration, and start the build.

docs/manual/manual.adoc

@@ -9,7 +9,7 @@ Buildroot {sys:echo $BR2_VERSION} manual generated on {localdate}
 
 The Buildroot manual is written by the Buildroot developers.
 It is licensed under the GNU General Public License, version 2. Refer to the
-https://git.buildroot.org/buildroot/tree/COPYING?id={sys:git rev-parse HEAD}[COPYING]
+https://gitlab.com/buildroot.org/buildroot/-/blob/master/COPYING[COPYING]
 file in the Buildroot sources for the full text of this license.
 
 Copyright (C) The Buildroot developers <buildroot@buildroot.org>