Add credentials collectors explanation pages

[shreyamalviya]

What does this PR do?

Fixes #4212

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

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 77.07%. Comparing base (dd956f4) to head (85099ce). Report is 1 commits behind head on develop.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## develop #4240 +/- ## ======================================== Coverage 77.07% 77.07% ======================================== Files 442 442 Lines 14135 14135 Branches 18 18 ======================================== Hits 10895 10895 Misses 3240 3240 ```

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[mssalvatore]

I expanded the explanation of Credentials Collectors:

I have two seemingly conflicting goals for this:

1. Provide context. More specifically, answer the question, "why steal credentials?" To that end, we also need to answer, "how are stolen credentials used?"
2. Keep this page concise and focused.

There may be a little bit too much detail about exploiters here, but I think it's important for users to understand how the stolen credentials are used. I think the solution would be to provide this information under Self-propagating Agent (Issue #4215) and just say, "for information about how stolen credentials are used ... blah blah ... see Self-propagating Agent". Since that page isn't ready yet, I think we should leave this text here. I've left a note in #4215 to extract/repurpose this text as part of that issue.

Similarly, instead of explaining exploiters, I'd rather just use the term "Exploiter" and link to the relevant documentation, but the relevant documentation doesn't exist yet. I've also left a note in #4213.