Fix 'latest' version label in Sawtooth doc TOC

Fix the confusion between "latest release" (link on sawtooth.hyperledger.org/docs) and "Latest" version label in the doc TOC.

• When you click on "Latest Release" from the Docs web page, the TOC shows the release number. This is a good thing.
• When you click on "Nightly master" from the Docs web page, the TOC shows "latest". This is confusing.

Consider a terminology change for the nightly master link and version label, along with including the word "Draft" in the label.

For an example, see the Sawtooth doc links at https://sawtooth.hyperledger.org/docs/