🌐 feat(i18n): overhaul translation system & add languages (#145)

Revamp the existing translation system, simplifying
management and adding several new languages. The new system reads from
TOML files in the `/i18n` directory and improves template structures.
It also enhances customisation options and robustness by providing
fallbacks and modularity.

- Implement a new, streamlined translation macro.
- Load translations from `/i18n` TOML files.
- Remove redundant configuration requirements.
- Refactor templates to align with new i18n system.
- Add support for Hindi, Japanese, Russian, Portuguese, Chinese,
  Italian, German, Ukranian, Korean, and French languages.
- Credit Thomas Weitzel (@thomasweitzel) for inspiration.

templates/macros/table_of_contents.html

@@ -1,4 +1,4 @@
-{% macro toc(page, header) %}
+{% macro toc(page, header, language_strings="") %}
 
 {%- set toc_levels = page.extra.toc_levels | default(value=3) -%}
 
@@ -8,7 +8,7 @@
 
 <div class="toc-container">
     {% if header %}
-        <h3>{{ macros_translate::translate(key="table_of_contents", default="Table of Contents") }}</h3>
+        <h3>{{ macros_translate::translate(key="table_of_contents", default="Table of Contents", language_strings=language_strings) }}</h3>
     {% endif %}
 
     <ul>