Update all links to the official Getting Started Guide - #3107

I found out that the link to "Binaries, strings, and charlists" official guide in en/lessons/basics/strings lesson leads to 404 page (https://elixir-lang.org/getting-started/binaries-strings-and-char-lists.html).

• I've updated the link to the new actual page (https://elixir.hexdocs.pm/binaries-strings-and-charlists.html)
• I've updated the link in all the translations
• I've updated all the links that were pointing to elixir-lang.org/getting-started to corresponding pages in elixir.hexdocs.pm
• I've self reviewed my PR and spotted BN translation had 2 #case links, I've replaced the latter with #cond. Even though I don't know any Bengali, I'm pretty sure the link should be #cond as it is located in the ## cond section