Improve "Contributing" page on documentation website

[chriscarrollsmith]

• Maybe add quickstart to index.qmd and render README from that file. And add links in the text of index.qmd to point to the other pages.

• We should be able to render to README.md by doing

quarto render docs/index.qmd README.md --to markdown

or something like that. Figure out the details and document them on the contributing page.

• Also add guidelines for opening issues, especially for reproducibility of bug reports.
• While you're at it, I just noticed that Authentication has two levels of headers and should only have one; fix this