Add extensive docs regarding the translation process

[Hans5958]

As of time of writing, the only piece of info that is useful for the translators is to joining the team. Other than that, it's a void. There's almost no information on how to do the translation/localization itself, leaving translators need to figure it out themselves.

If there is a better documentation regarding the localization process itself, it would make the process more efficient and effective. Low-quality translations can also be avoided. Potential new contributors can also read the guide first to have a picture of what we expect/want regarding the translations.

Settled discussions such as ScratchAddons/ScratchAddons#5187 and those on the Discord server can also be added for future reference.

Some examples:

• Scratch: https://github.com/LLK/scratch-l10n/wiki/Guide-for-Scratch-Translators
• Scratch Addons website: https://github.com/ScratchAddons/website-v2/wiki/Translating

PS. This has been raised multiple times on the Discord server, but I finally had time to write this as an issue.

[WorldLanguages]

I think we should begin by changing the https://scratchaddons.com/translate page slightly. That's probably the first page potential translators read (after clicking "help translate" inside the extension).
That page could also be added to the website footer. I'm not sure if we ever considered it.

[Hans5958]

Scratch's Getting Started page can be referred for that. That page also linked the guidelines on the bottom.

[WorldLanguages]

There's also the Help Translate TurboWarp page