docs: publish master docs to latest path

[travisn]

Description of your changes:
Rename the master docs to latest. This will be easier to understand for users who are new to the project. The url will also contain latest, which will be more clear to reference instead of the master path. This also follows the pattern of the ceph docs site.

Should be merged shortly after rook/rook.github.io#115

Checklist:

• Commit Message Formatting: Commit titles and messages follow guidelines in the developer guide.
• Skip Tests for Docs: Add the flag for skipping the build if this is only a documentation change. See here for the flag.
• Skip Unrelated Tests: Add a flag to run tests for a specific storage provider. See test options.
• Reviewed the developer guide on Submitting a Pull Request
• Documentation has been updated, if necessary.
• Unit tests have been added, if necessary.
• Integration tests have been added, if necessary.
• Pending release notes updated with breaking and/or notable changes, if necessary.
• Upgrade from previous release is tested and upgrade user guide is updated, if necessary.
• Code generation (make codegen) has been run to update object specifications, if necessary.