[lldb][docs] Resurrect the information on adding a new language (#109427)

This got deleted in e078c9507c3abb4d9bb2265c366b26557880a3e3, I presume
accidentally, because it didn't have a corresponding rst file for it.

So I've brought it back and converted it into Markdown. The content
remains accurate, from what I know at least.

It's a bit "now draw the rest of the owl" but if nothing else, it gives
you a bunch of important classes to go and research as a starting point.

You can see the original content here:
https://github.com/llvm/llvm-project/blob/5d71fc5d7b5ffe2323418a09db6eddaf84d6c662/lldb/www/adding-language-support.html

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