Document an overview of how docs are translated

**Live page: **https://handbook.gitlab.com/handbook/marketing/localization/tech_docs_localization/

Draft doc for collaboration here.

Structure:

1. Overview: what's in scope and what isn't, some specific terms that we're using to name key elements of the program
2. Localization architecture: description of the tech docs fork, what's the Argo-GitLab integration, Translation MRs (what are they, labels and typology), process of upstream merges
3. Translation workflow: overview and summary of all the steps of the workflow, how are AI translations generated and QAed, what is a translation request
4. Tech docs internationalization: locale folder structure, internationalization features contributions to make Hugo multilingual, tech docs pipelines
5. Localization UX features: Contributing to translated documentation, EN content in the localized page
6. Known limitations and future work: 3rd party UI labels vs GitLab UI translation treatment, markdown rendering edge cases, limitations on AI QA checks, search capabilities (Elasticsearch),