Add all missing English translations of MD documents

[hoijui]

... using Google translate + manual MD syntax fixing.

And then add some more, minor fixes and improvements in the English build-guides,
while also unifying a few sections among the guides.

[frioux]

BTW I used this guide and it was very helpful. The only part that wasn't very clear was

Insert the pin header into the pin socket for OLED first, and then solder the pin header and OLED module. At this time, the OLED module is easy to float, so be careful not to float it while pressing it with your finger.

I guessed that float in this context means raise the through holes such that it's no longer flush, but I still am not really sure what it meant. Anyway this would be great if it were merged!

[tsndr]

Thank you so much ❤️

@foostan Can you please merge this?

[foostan]

@hoijui Thank you for your commitment! There has been a lack of an EN version for a long time, but I'm sure many people will be happy with this. I'll fix the details later, but first I'll merge it. Thank you very much!