🐛 fix(header): check if translations exist while building tags

[welpo]

Description

This PR aims to resolves #158, the build-stopping bug when pages are missing translations for all config languages.

Changes Made

1. Modified the Tera template to conditionally check for the existence of translated pages or sections before invoking get_page or get_section.
2. Utilised load_data with required=false as a workaround to check for the existence of a .md file, thus preventing build errors.

{# Check if the translated page or section exists #}
{%- set translation_exists = load_data(path=translated_filename, required=false) -%}

{# Get the page or section details #}
{%- if translation_exists -%}
    {%- if page.relative_path -%}
        {%- set translated_page = get_page(path=translated_filename, metadata_only=true) -%}
    {%- else -%}
        {%- set translated_page = get_section(path=translated_filename, metadata_only=true) -%}
    {%- endif -%}
{%- endif -%}

Points to Consider

• The use of load_data here is primarily a workaround due to the lack of a direct function to check for file existence in Zola (see this discussion). Therefore, this solution might not be the most idiomatic or efficient, but it serves the purpose of resolving the build-stopping issue.