Improve open graph meta tags on 18F home page

[michelle-rago]

The 18F home page has missing information in open graph tags which impacts how the 18F site is previewed on social media. You can see the impact of this missing info on https://opengraph.dev/panel?url=https%3A%2F%2F18f.gsa.gov%2F.

Also, all pages should have unique HTML page titles and meta page descriptions. These are federal website standards.