Modify notebook headers to match file names; update index.md

[tomdonaldson]

I modified the top-level header of each content notebook md file to match the file names. I also updated index.md to give a little more organization to the list of notebooks.

Apologies for my VS Code editor removing some trailing whitespace and changing the bullet character. I also inadvertently modified some of the notebook metadata ( like Python and jupytext version).

Some possible future items:

• I wish the sidebar retained more of the hierarchical structure and depth of the main table of contents, but I couldn't quickly see how to do that.
• I noticed the the VO Tables reference notebook contains some confusing things. I guess it's helpful if you're creating a table upload, otherwise I would just remove it for now.