fix(doc): Install sphinx_rtd_theme explicit

While building the docs on readthedocs.org (RTD) the package sphinx_rtd_theme is explicit installed. The reason is a change on the side of RTD.

.readthedocs.yaml

@@ -10,7 +10,13 @@ build:
   os: ubuntu-22.04
   tools:
     python: "3"
-
+  jobs:
+    # Workaround: See PR #1554 for details.
+    # When migrating to use a pyprojects.toml file switch from this
+    # workaround to the use of "python: install: extra_requirements..."
+    # See also: https://docs.readthedocs.io/en/stable/config-file/v2.html#packages
+    post_create_environment:
+      - python -m pip install sphinx_rtd_theme
 # Build documentation in the docs/ directory with Sphinx
 sphinx:
   configuration: common/doc-dev/conf.py
@@ -19,5 +25,8 @@ sphinx:
 # We recommend specifying your dependencies to enable reproducible builds:
 # https://docs.readthedocs.io/en/stable/guides/reproducible-builds.html
 # python:
-#   install:
-#   - requirements: docs/requirements.txt
+#  install:
+#  - method: pip
+#    path: .
+#    extra_requirements:
+#    - foo

common/doc-dev/conf.py

@@ -160,7 +160,7 @@
 
 # If not '', a 'Last updated on:' timestamp is inserted at every page bottom,
 # using the given strftime format.
-html_last_updated_fmt = '%b %d, %Y, %H:%M'
+html_last_updated_fmt = '%b %d, %Y, %H:%M (%Z)'
 
 # If true, SmartyPants will be used to convert quotes and dashes to
 # typographically correct entities.