Merge sphinx-docs (CLI i18n + Sphinx docs) into main

# Conflicts:
#	.github/workflows/ci.yml
#	docs/cli.md

tests/conftest.py

@@ -0,0 +1,13 @@
+import pytest
+
+
+@pytest.fixture(autouse=True)
+def _force_source_locale(monkeypatch: pytest.MonkeyPatch) -> None:
+    """Keep CLI output in the source language during tests.
+
+    ``nexus-cli`` activates a locale from ``NEXUS_SYNC_LANG`` / the system
+    locale, so assertions on English output would break on a machine whose
+    locale is, e.g., Russian. Pinning to a language with no catalog makes
+    gettext fall back to the source strings regardless of the dev's environment.
+    """
+    monkeypatch.setenv("NEXUS_SYNC_LANG", "en")