Add banner to GitHub Pages site indicating that it is not a production site

[pkalita-lbl]

Currently we have a web version of the app hosted by GitHub Pages and accessible at https://fieldnotes.microbiomedata.org. This was primarily useful to demonstrate work to non-developers before we were able to build and deploy native versions of the app. And because it was a testing/demoing site it was (and still is) connected to the dev nmdc-server API. Now we do have native builds of the app that people are testing. They point to the prod nmdc-server API. Additionally, since the GitHub Pages versions rebuilds with every commit to main it may have changes that haven't been deployed to native versions yet. I don't expect people to be using that site for much of anything, but we should still somehow warn people of those differences if they stumble upon it.

[pkalita-lbl]

See also: #167