Add Debug Info section

[Walnut356]

Adds a pretty substantial debug section. There are pages for GDB, but those contain mostly links to GDB's documentation because I don't know a ton about it at the moment. I'm currently working on the debuginfo test rewrite, which is starts with tests for LLDB. Once I get around to writing tests for GDB, I will end up sufficiently knowledgeable to fill out the GDB sections (if they haven't been filled out already by then).

There are 2 somewhat questionable changes:

1. I have included the raw PDF file of the CodeView 1.0 specification, which is linked to in the intro page. AFAICT Microsoft does not host it anywhere. The sources that do host it have dubious longevity. The direct source of this PDF had their website die for a short while just in the last year. I think it's important that this is preserved and accessible, I used it extensively when working with PDB debug info. If you'd rather not have the full PDF included, I can go ahead and take it out.
2. The old debug info page was simply moved into the new debug info section (and labelled as lecture notes). Some of the info is outdated, some of it isn't. I didn't feel super comfortable chopping it up and interspersing it in what I wrote since it's directly from a lecture.

Probably resolves #1661

[rustbot]

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[ehuss]

This is amazing!

May I suggest that when this is merged that you share it, maybe in the #t-compiler Zulip channel to let people know it exists? I imagine there will be people who will find it interesting (even if they aren't directly involved with it).