Update all community email management references for new service

[skliper]

Checklist (Please check before submitting)

• I reviewed the Contributing Guide.
• I reviewed the cFS README.md file to see if the feature is in the major future work.
• I performed a cursory search to see if the feature request is relevant, not redundant, nor in conflict with other tickets.

Is your feature request related to a problem? Please describe.
Community email system is transitioning (currently May 8th target), which changes join/leave process.

Describe the solution you'd like
Update instructions in sync with system change over:

To subscribe to [INSERT LIST NAME]: 
Send an e-mail to [email protected] (no subject or text in the body is required).

To unsubscribe to [INSERT LIST NAME]:
Send an e-mail to [email protected] (no subject or text in the body is required).

Having trouble subscribing or unsubscribing from a list?
Get help by sending an e-mail to the list owner at [email protected]

Describe alternatives you've considered
#207

Additional context
Note a web interface is pending. Not clear if it will support review of archives by members.

Requester Info
Jacob Hageman

[astrogeco]

I think #258 fixed this. What are we missing?

[skliper]

Just tested the methods described in #258 and it works for me. Linked and closing.