Updating README and cleaning up documentation

[StevenBucher98]

PR Summary

Pass at README and other docs in the repo in preparation for public preview. Still working on this

PR Context

Modified root README, agent README and including doc links that will be live next week.

[daxian-dbw]

Wait, you haven't addressed https://github.com/PowerShell/ProjectMercury/pull/286#discussion_r1837077169. Can you make v7.4 a hyper link to the 7.4.6 release page?

[daxian-dbw]

Size of the logo seems too small