[i18n & ja] Improved fallback config and normalize en doc links

[chalin]

• Contributes to Improved i18n support #4467
• Adjusts htmltest ignore rules to be more inclusive of i18n pages, without having to name localization languages (well, as long as they are two letters for now :).
• Normalizes links in en pages to make localization easier
• Adjusts (improves) fallback config for ja and zh to be more inclusive

Preview: https://deploy-preview-4627--opentelemetry.netlify.app/ja/docs/

Followup:

• Move images from under content/en/docs/demo/screenshots into static to ensure that they are shared(?)

[chalin]

Sigh, no this isn't ready. I got over ambitious with the fallback config by trying to cover as many sections as possible. Need to backtrack. Setting back to draft until then 🤷🏼‍♂️

[chalin]

This is ready for review once again.