Add assembly documentation for PowerPC platform (#6665)

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Hello all, I've added assembly documentation for the PowerPC platform
with this PR.

There's a couple of notes to keep in mind, mostly regarding automatic
processing:

1. Unfortunately, IBM made their documentation site very difficult to
automatically scrape. It uses dynamic rendering with a documentation API
that's protected by cookies, requiring the use of Selenium to scrape it.
2. While it is possible to scrape the documentation pages, it's
maddeningly difficult to do so, because the structure of the pages is
not guaranteed.
- Some pages have invisible elements in their headers that make matching
elements very difficult.
     - Some pages are missing entire sections.
- Some pages are written without an introduction for the instruction
mnemonic, making the resulting documentation impossible to understand.
- Some pages, specifically for instructions without arguments, are
missing the table that lists the mnemonics out in order.
- I could go on, but I abandoned trying to do automatic processing after
running into this many roadblocks.
3. The automatic processing script is available in the docenizers
directory, but since it's not complete, feel free to ask to remove it.
4. I couldn't test the actual mnemonics appearing the editor, because my
local test setup kept trying to map the PowerPC instructions into AMD64
documentation, with predictably unusual results.
5. I didn't include AltiVec instructions, because there isn't any
webpage that documents them; they are only documented in a PDF file
provided by NXP.

Please let me know if there's anything else I need to do for this to
work. Thank you so much for your wonderful service!

CONTRIBUTORS.md

@@ -148,3 +148,4 @@ From oldest to newest contributor, we would like to thank:
 - [Matthew Guidry](https://github.com/mguid65)
 - [Adam Thibert](https://github.com/adamaq01)
 - [Nazım Can Altınova](https://github.com/canova)
+- [Nicholas Hubbard](https://github.com/nhubbard)