added introduction

[Nancy9ice]

Overview:
Closes this Google Season of Docs Task:
Add the readthedocs landing page as the Introduction in the Table of Contents

What problem does this address?
This adds the landing page to the table of contents to ease navigation

Testing
How did you make sure this worked? How can a reviewer verify this?
I tested it by running make make docs-build

@aesharpe @zaneselvans @e-belfer

[e-belfer]

Can we call this "About PUDL" instead of "Introduction"?

Rather than creating a blank index page and a new Introduction page, I think we need to add the index page to the TOC.

See: https://stackoverflow.com/questions/16123951/how-do-i-include-the-homepage-in-the-sphinx-toc

[e-belfer]

Suggested change

	Introduction <Introduction>
	About PUDL <self>

This will create a reference to the page in the table of contents without needing to create a new page or having an empty index.