Update developer install documentation, add user authentication content

[mradamcox]

@karenmajewicz This branch was originally made from your add-documentation branch, but I assume it will still need to be rebased (or something) before it can be merged. Just wanted to get a PR in at this point for review. Happy to close and remake once the docs reorganization is more settled.

The new content includes:

• Update developer docs page, now called "For Developers" in Install section.
• Add architecture diagram within that section
• Move the piece about using an external Solr instance to the production section
• Add page about user authentication (this could be fleshed out more, but it's a start).

[mradamcox]

Ok the only conflict was in the mkdocs.yml. I've resolved that so this could be merged after review.