revamp homepage

[adw96]

Woah, we need a better-looking homepage. We made the radEmu page look pretty nice and accessible... need the same for happi

TODOs

• A LINK TO THE PUBLISHED PAPER ASAP
• logo
• a "If you are a microbial ecologist or bioinformatician, some of the things that you may like include..."
• a "If you are a statistician, some of the things that you may like include..."
• usage in the R style (as distinct from install)

@MariaAVC Can you please start a PR implementing the above with specific requests for me to add content? 🙏

[MariaAVC]

I've addressed this in PR #59