Restructure utilities and improve documentation of them

[shaunagm]

Most of our helper code can be found in parsons.utilities however almost none of it is in the documentation. We should update the docs!

There's also, separately, a tools folder containing a single file with credentials tools. I think it makes sense to move the credentials file into utilities. If there's a chance users are calling this directly, we should probably replace it with a file that imports from the new location in utilities and logs a deprecation warning, then come back and delete it in like 6 months. If we think it's only being used internally we can just update our own calls and delete the file+folder.

While we are doing this, it's worth considering:

• structuring the utilities folder to give it subfolders, to help people reason about and find different classes of utilities
• adding additional documentation/writing a guide to adding utilities that helps people discover and add to the utilities
• rename etl to table or something slightly more descriptive (while we are moving/renaming things, might as well handle other issues, although I don't think this is high priority)

Also I just want to mention #724 (which I am now closing) which I think is a bad choice as written but contains some useful code and comments for however we decide to approach utilities/helpers.

[shaunagm]

(I tried to tag this issue via a comment in a PR but I don't think Github is surfacing that. The PR is this one, there's a helper method in there) https://github.com/move-coop/parsons/pull/896/