Docs: Add documentation for the SNMP exploiter

[cakekoa]

What does this PR do?

Fixes part of #3234.

Add documentation for the SNMP exploiter

PR Checklist

• Have you added an explanation of what your changes do and why you'd like to include them?
• Is the TravisCI build passing?
• Was the CHANGELOG.md updated to reflect the changes?
• Was the documentation framework updated to reflect the changes?
• Have you checked that you haven't introduced any duplicate code?

Testing Checklist

• Added relevant unit tests?
• Do all unit tests pass?
• Do all end-to-end tests pass?
• Any other testing performed?

  Tested by {Running the Monkey locally with relevant config/running Island/...}

• If applicable, add screenshots or log transcripts of the feature working

[mssalvatore]

v3 is still vulnerable to CVE2020-15862, right? It's just that authenticating is harder. This language makes it seem like v3 is not vulnerable.

After clarifying the above, we may want to add an explicit note regarding which protocol versions we support in this exploiter.