Fix translation of html pages

[StanFromIreland]

There two issues, firstly some languages were not compiled and therefore don't load (e.g. Polish), and secondly, we hijacked the gettext for html files with python-docs-theme.po, resulting in messages in sphinx.po not loading (e.g. Greek).

Complete preview, built with translations: https://stan.ulbrych.org/preview/theme-translation/ (vs. current docs)

Compilation issue

We now get:

compiling catalog python_docs_theme/locale/zh_TW/LC_MESSAGES/python-docs-theme.po to python_docs_theme/locale/zh_TW/LC_MESSAGES/python-docs-theme.mo compiling catalog python_docs_theme/locale/tr/LC_MESSAGES/python-docs-theme.po to python_docs_theme/locale/tr/LC_MESSAGES/python-docs-theme.mo compiling catalog python_docs_theme/locale/pt_BR/LC_MESSAGES/python-docs-theme.po to python_docs_theme/locale/pt_BR/LC_MESSAGES/python-docs-theme.mo compiling catalog python_docs_theme/locale/sv/LC_MESSAGES/python-docs-theme.po to python_docs_theme/locale/sv/LC_MESSAGES/python-docs-theme.mo compiling catalog python_docs_theme/locale/pl/LC_MESSAGES/python-docs-theme.po to python_docs_theme/locale/pl/LC_MESSAGES/python-docs-theme.mo compiling catalog python_docs_theme/locale/ja/LC_MESSAGES/python-docs-theme.po to python_docs_theme/locale/ja/LC_MESSAGES/python-docs-theme.mo compiling catalog python_docs_theme/locale/zh_CN/LC_MESSAGES/python-docs-theme.po to python_docs_theme/locale/zh_CN/LC_MESSAGES/python-docs-theme.mo compiling catalog python_docs_theme/locale/es/LC_MESSAGES/python-docs-theme.po to python_docs_theme/locale/es/LC_MESSAGES/python-docs-theme.mo compiling catalog python_docs_theme/locale/el/LC_MESSAGES/python-docs-theme.po to python_docs_theme/locale/el/LC_MESSAGES/python-docs-theme.mo 

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📚 Documentation preview 📚: https://python-docs-theme-previews--263.org.readthedocs.build/

[rffontenelle]

LGTM

[StanFromIreland]

Added type hints per documentation.