Add more language about how .gov domains are free

[kristinacyin]

Issue description

We've received quite a few emails asking about the cost of having a .gov domain. @michelle-rago suggested that

Maybe we should add it at the top of https://get.gov/domains/before/ (which gets a lot of traffic) and perhaps something in the horizontal bar on https://get.gov/domains/

Acceptance criteria

Add additional language about how .gov domains are free to eligible organizations...

• draft text
• At the top of get.gov/domains/before/
• In the horizontal bar on get.gov/domains/

Additional context

Slack thread

Links to other issues

Note: Also suggested to add a related illustration #ill

[kristinacyin]

I'm closing this for now since this will probably be addressed by updates to the contact form #289