Mention the valgrind Side Channel Analysis

[reneme]

The "audit_method" document has a few mentions of the valgrind-based side-channel analysis tests. Especially after randombit/botan#4182 was added, it might be worthwhile to add a description to the testspec. I'd envision the content to go roughly like that:

• How does the valgrind-based CT checks work?
  • uninitialized memory being marked and tracked by valgrind
  • errors produced when such memory is used in branches or to access memory
• Limitations of the approach
  • needs explicitly maintained annotations in the implementation
  • does not capture variable-time execution of machine instructions (such as KyberSlash)
• The CT-selftest in CI:
  • explicitly provoking errors to ensure the valgrind setup as well as the CT::poison helpers work as expected
• List of algorithms that are annotated