Add credentials collectors' reference pages

[shreyamalviya]

What does this PR do?

Fixes #4210

PR Checklist

• [x] Have you added an explanation of what your changes do and why you'd like to include them?
• [x] 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?~
• [x] Any other testing performed?

  Tested with Hugo

• [x] 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 77.08%. Comparing base (3fa4467) to head (39e1915).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## develop #4229 +/- ## ======================================== Coverage 77.08% 77.08% ======================================== Files 442 442 Lines 14137 14137 Branches 18 18 ======================================== Hits 10897 10897 Misses 3240 3240 ```

:umbrella: View full report in Codecov by Sentry.
:loudspeaker: Have feedback on the report? Share it here.

[cakekoa]

Should we bring over the Description section from the old reference pages? Configuration alone is pretty bare. What is the Chrome credentials collector? How does it work? What sort of environment will it work in?

[shreyamalviya]

Should we bring over the Description section from the old reference pages? Configuration alone is pretty bare. What is the Chrome credentials collector? How does it work? What sort of environment will it work in?

All of that, IMO, should be in the pages under features/, not here. (#4212)