Revised translation process doc

While becoming more familiar with Bitcoin Core, I came across `doc/translation_process.md`. I felt it could use a refresh, so I've rewritten it to hopefully make it more clear for first time readers, while also being useful as a reference guide. I've kept all previous information. The new stuff is mostly just reformatting, and adding additional details.

If someone could please review it technically, to catch any mistakes.

Due to the diff being so messy, here is a link to the original for reference and comparison. 
[doc/translation_process.md (to be replaced)](https://github.com/bitcoin/bitcoin/blob/0286fe5b3be20d0c27c6a71fa1a222166bccfd65/doc/translation_process.md).
The path has since changed to `src/`. This needs to be updated.