This PR configures the site's metadata and favicon with additional Gatsby plugins:
Use gatsby-plugin-canonical-urls to ensure that the www version of the site is exclusively indexed by Google.
Use gatsby-plugin-sitemap to generate a sitemap
Use gatsby-plugin-robots-txt to generate a robots.txt file.
Configure the gatsby-plugin-manifest plugin to set our favicon and use branded colours in progressive web app situations. Hopefully, we'll have a real favicon once the Rubin Obserservatory identity and branding is released, but for now, we're using a rendering of the observatory from https://gallery.lsst.org/bp/#/folder/2334407/51647471
Customize the description metadata for pages built via the docSeries template.