Ensure manual defaults to latest minor version

- Added an application-level config file that will set the default version for
  the manual route.
- Added a target to Makefile to update the above file based on the version
  specified; this target gets invoked when we prepare the site for a release.
- Updated the redirects to pull the version from the above config file if
  /manual or /manual/ are requested.
- Added a "serve" target to Makefile to make serving the site via the built-in
  web server simpler; allows passing a PORT variable if desired.
Merge branch 'hotfix/default-manual-version'