Add modding translation instruction to translation.md inpired by #6483.
Mostly is copied from @SomeTroglodyte 's comment and add some of my own translation expeirence.
There might be some grammatical mistakes in this doc.
* Fixed the links in the docs
* Fixed broken tables
Tables without a line break above are not treated as valid tables
* Removed redundant table of contents and formatted some bulleted lists
* The image in Regions.md is now properly displayed
* Standardized list indentation, line spacing, table header rows, and JSON code blocks
* Standardized headers
* Fixed remaining broken links, edited pseudo-footnotes
