Stop the privacy plugin from downloading the repology.org badges (#5523)

repology.org refuses requests coming from GitHub Actions runners. The
Material privacy plugin logs the failed download as a warning, but then
raises FileNotFoundError when it reads back the cache entry it never
wrote, so ci_test_build_documentation aborts the whole documentation
build. develop and every open pull request are currently red because of
this.

The badges are fine for readers -- repology only rejects non-browser
clients -- so exclude them from the plugin's asset self-hosting and keep
them as external references. All other external assets (fonts, shields,
CDN files) are still downloaded and self-hosted as before.

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

docs/mkdocs/mkdocs.yml

@@ -394,7 +394,14 @@ plugins:
         - http://nlohmann.github.io/json/*
         - https://nlohmann.github.io/json/*
         - mailto:*
-  - privacy
+  - privacy:
+      # repology.org refuses requests from GitHub Actions runners, which made
+      # the privacy plugin abort the whole build when it could not download the
+      # package badges (the fetch fails, then reading the missing cache entry
+      # raises FileNotFoundError). Readers' browsers are served normally, so
+      # leave these badges as external references instead of self-hosting them.
+      assets_exclude:
+        - repology.org/*
   - llmstxt:
       markdown_description: >
         JSON for Modern C++ is a C++11 header-only library implementing a JSON