Centralize and elevate multilingual documentation

[jmooring]

A) Centralize. Aspects of creating a multilingual site currently reside in multiple sections.

B) Elevate to its own section. Placing it all on one page is too much.

C) Initial thoughts on topics (in no particular order)

1. Overview
2. The language key, RFC 5646, and what it controls (see this comment)
   • URL prefix
   • Translation
   • Localization (dates, currencies, etc.)
   • Collation
   • Template lookup order (including shortcodes and render hooks)
3. Configuration (including description of language-specific site configurations overriding root table such as [params], [permalinks], [menus])
4. Config as a directory in a multilingual site
5. Site variables (e.g., .Site.Language.LanguageCode, etc.)
6. Sites variables (e.g., .Sites, .Sites.First.Home.RelPermalink, etc.)
7. Page variables (e.g., .Page.Language.LanguageCode, etc.)
8. Content structure
9. Front matter (title, slug, url, translationKey)
10. Language specific permalinks
11. Language specific site parameters
12. Translation tables (simple translations, pluralized translations, subkeys, etc.)
13. Translation table fallbacks
14. Finding missing translations
15. Example of creating a translation table without values via external script
16. Menus (configuration, front matter, template variables, example template)
17. Taxonomies
18. Filling in the gaps (pages: lang.Merge and content mounts)
19. Filling in the gaps (page parameters: partial)
20. Language specific templates (including shortcodes and render hooks)
21. Multilingual multihost projects
22. Simple example of a language switcher
23. Creating new content with hugo new
24. Functions (e.g., lang.Translate and aliases, lang.FormatCurrency, ref, relref, .Page.Ref, .Page.RelRef, absLangURL, relLangURL, etc.)
25. Shared resources (e.g., image in page bundle)
26. Sitemaps
27. Targeting a specific language with [[cascade]] in site configuration
28. Language specific 404 templates (use alternate approach by creating content pages and custom template)
29. Language specific data files (the data directory)
30. Reserved words. This was resolved with #2437. See https://gohugo.io/functions/lang/translate/#reserved-keys.
31. De-duplication of page resources on multilingual, single host sites, and the automatic enablement of the embedded link and image render hooks