Small cleanup changes

This PR includes:

* several minor changes that clean up the root directory of the website
* a markdown file to include text in the grid page (although new figures in the grid still need to be added by editing the html file)
* avoids tracking the `_site` folder, which gets built on the server by github anyway and can be source of conflicts between different user commits
* avoids tracking .swp files so those open in vim are no problem
* cleans some of the html page so they don't produce warnings in jekyll