Add splash page with some context

[jdangerx]

Overview

What is the problem we're solving? For very simple items, this can be encapsulated in the success criteria.

Success Criteria

How will we know that we're done?

The default landing page explains, concisely and with links:

• what PUDL is
• what are some questions we can answer
• what are some useful datasets we provide

Next steps

Preview Give feedback

1. factor out the navbar, styles, etc. into a base template
2. make a new jinja template for homepage
3. make / route render the homepage
4. write content!!!!
5. add /search to navbar

[zaneselvans]

A pointer to docs / examples on how to work with the Perspective datagrid could also be helpful -- it seems like there's a lot in there!