CI: Remove building docs on macOS

Those CI workflows' intentions are mostly to run the link checker, so
it's not too much connected to covering operating systems.

By removing the checks for macOS, the process will relax pressure on
downstream resources, aiming for making it more unlikely to hit rate
limiting measures.

Author: amotl

.github/workflows/docs.yml

@@ -21,7 +21,7 @@ jobs:
 
     strategy:
       matrix:
-        os: [ubuntu-latest, macos-latest]
+        os: [ubuntu-latest]
 
     steps:
       - name: Acquire sources