add robots.txt and sitemap.xml

[derekeder]

Before we get to #196, we agreed last night to prioritize adding a robots.txt and sitemap.xml to the site so search engines like Google and Bing can more easily crawl our pages, especially our new power plant pages.

I did a little digging around and Gatsby has a plugin for configuring these files: https://www.gatsbyjs.com/plugins/gatsby-plugin-robots-txt/

However, after installing it locally, the expected files are not being generated. I believe I had a similar issue with this plugin on the predecessor to this app, https://greenjobsdata.com/.

Given the relative simplicity, I think I'll skip the plugin and manually configure these pages.