Devops: Add --color option to linkcheck

This helps with spotting broken links in the output. Without the
coloring it is difficult to sift through all the output. For the CI
workflow, the `-q` flag is also added such that only broken links are
printed for easy debugging.

Author: sphuber

.github/workflows/ci.yml

@@ -24,4 +24,4 @@ jobs:
         pip install --upgrade pip
         pip install -rrequirements.txt
     - name: Run linkcheck
-      run: sphinx-build -b linkcheck docs/ docs/_build/linkcheck
+      run: sphinx-build -b linkcheck docs/ docs/_build/linkcheck --color -q

tox.ini

@@ -23,4 +23,4 @@ deps = -rrequirements.txt
 allowlist_externals =
     rm
 commands_pre = rm -rf docs/_build/htlinkcheckml
-commands = sphinx-build -b linkcheck docs/ docs/_build/linkcheck
+commands = sphinx-build -b linkcheck docs/ docs/_build/linkcheck --color