Add Spanish (es) translation

Description

Would you accept a PR adding a Spanish translation for the template? A few things I'd like to confirm before opening it:

1. Locale code. I'd lean toward es (which is what Django core itself ships, as well as django-allauth). The existing locales here are country-specific (fr_FR, pt_BR), so happy to use es_ES instead if you prefer that.
2. Tone. Plan is to keep it neutral and use infinitives where possible (e.g. Iniciar sesión, Cerrar sesión), to avoid the tú/usted question and stay friendly to Spanish speakers from any region.
3. Format. The README under locale/ suggests running makemessages --no-location, but the current fr_FR/django.po and pt_BR/django.po both include line locations. I'll match the existing files unless you'd rather migrate to --no-location.

Rationale

Spanish is currently missing from the available locales (en_US, fr_FR, pt_BR), and it's one of the most widely spoken languages. For context: #4025 (2022) was closed because no translations were bundled back then. Since then fr_FR and pt_BR have been merged, so I assume the policy has shifted to welcome locale contributions — but happy to drop this if that's not the case.