Changes related to the translation pages

[IlDucci]

The following commit/PR attempts to do minor tweaks and improvements over all the translation-related content:

• Renamed the page titles on the index bar to be more clear.
• Edited the new-translations.md page in order to dissuade people from directly editing the msg_hash_XX.h files (since Crowdin will automatically overwrite the contents and any translations added via push/PR could require finetuning by existing translators), as well as changing its purpose to teach how to add a new language.
• Fixing a missing line-jump in new-translations-crowdin.md.
• Re-cut the button icons so they are more proportionate to the texts.
• Edited every screenshot that contained the LanguageTool icon (so people don't ask why is that thing not in their browsers, sorry about that) to remove them.
• Did a PNG optimization pass to the entire folder. The PNG sizes have been halved. (I have used the FileOptimizer program, default settings, removed all the metadata, avoided a lossy compression)