Fix antsibull-docs error if unexpected plugin type is documented

[bcoca]

fixes for

00:46   File "/root/ansible/docs/docsite/../../hacking/build-ansible.py", line 103, in <module>
00:46     main()
00:46   File "/root/ansible/docs/docsite/../../hacking/build-ansible.py", line 92, in main
00:46     retval = command.main(args)
00:46   File "/root/ansible/hacking/build_library/build_ansible/command_plugins/docs_build.py", line 235, in main
00:46     return generate_base_docs(args)
00:46   File "/root/ansible/hacking/build_library/build_ansible/command_plugins/docs_build.py", line 121, in generate_base_docs
00:46     return antsibull_docs.run(['antsibull-docs', 'stable', '--deps-file', modified_deps_file,
00:46   File "/root/.ansible/test/venv/sanity.docs-build/3.10/9e08beb0/lib/python3.10/site-packages/antsibull/cli/antsibull_docs.py", line 411, in run
00:46     return ARGS_MAP[args.command]()
00:46   File "/root/.ansible/test/venv/sanity.docs-build/3.10/9e08beb0/lib/python3.10/site-packages/antsibull/cli/doc_commands/stable.py", line 447, in generate_docs
00:46     return generate_docs_for_all_collections(
00:46   File "/root/.ansible/test/venv/sanity.docs-build/3.10/9e08beb0/lib/python3.10/site-packages/antsibull/cli/doc_commands/stable.py", line 319, in generate_docs_for_all_collections
00:46     remove_redirect_duplicates(plugin_info, collection_routing)
00:46   File "/root/.ansible/test/venv/sanity.docs-build/3.10/9e08beb0/lib/python3.10/site-packages/antsibull/docs_parsing/routing.py", line 275, in remove_redirect_duplicates
00:46     plugin_routing = collection_routing[plugin_type]
00:46 KeyError: 'filter'
00:46 make: *** [Makefile:205: base_plugins] Error 1

[felixfontein]

@bcoca could you please add a changelog fragment?