Restore downloads section

[garfieldnate]

We have dozens of agents, tools, etc. that were downloadable from the old website. The last couple of Soar releases have included all of them in the zip, which I don't think is ideal. All of the explanations, author names, etc. are gone.

[garfieldnate]

If we need download links to match their previous URLs, we can use https://github.com/mkdocs/mkdocs-redirects.

[garfieldnate]

• Agent development tools
• Agents
• Domains
• Examples and unsupported
• Soar (archive, see Add links in archives download page #31)