x/pkgsite: improve documentation on worker architecture and how documentation is rendered

[julieqiu]

Add the following:

• Describe and link to worker endpoints in server.go to doc/worker.md
• Describe how packages are added, and link to module proxy documentation:
  • https://golang.org/cmd/go/#hdr-Module_proxy_protocol
  • https://proxy.golang.org
• Describe how source links are generated and link to https://github.com/golang/gddo/wiki/Source-Code-Links
• Describe what happens during zip processing
• Add a script to seed local database for initial setup

/cc @jba @jamalc @AceroM @shaqque