Update test-helpers docs section

[hellais]

Currently the test-helpers section of the docs is misleading. The following things should be clarified:

• The web connectivity test helper is used in support of the web_connectivity test. It's used for carrying out control measurements and is accessed via a HTTPS API. It's not the only test helpers though.
• What are other test helpers that exist
• Update the section on the rotation script to mention that now it's deployed using a different pattern on digital ocean