[docs] Update the static analyzer bug reporting page

I happened to notice this page (https://clang-analyzer.llvm.org/filing_bugs.html)
was a bit stale, so I've updated and simplified it a bit.

* The page is now explicitly in UTF-8 (NFC)
* We no longer talk about Bugzilla and Apple's bug reporting tools, but
instead link to GitHub's issues page
* We now link to the general LLVM documentation on how to submit a bug

Differential Revision: https://reviews.llvm.org/D158360