Make GitHub Discussions more visible as the default place to get support with Unit

[danielledeleo]

This page is currently the only place where it is mentioned. It should be highlighted more clearly in the first paragraph here. Beyond that, I think there should be a more visible link elsewhere, possibly on the home page, or all pages.

Things to consider for this task and where users should be directed:

GitHub Issues

• bug reports
• feature parity (support for new PHP releases, HTTP/2, etc)

GitHub Discussions

• general support (how/can do I do X)
• feature requests (please add support for Lua)