Sharethrough: Schain bidfloor adapter docs update

[madma]

Type of change

• Feature
• Does this change affect user-facing APIs or examples documented on https://prebid.org?

Description of change

• Documentation change to indicate that Sharethrough's bid adapter has been updated to support the schain module and to check for and send the bidfloor parameter when building bid requests.
• pubgrowth.engineering@sharethrough.com
• See related prebid.js PR: Support schain module and send bidfloor param in Sharethrough adapter Prebid.js#4271

[madma]

hi @bretg -- i just pushed another update to this PR, a small change to our documentation. also, just wanted to note that the corresponding prebid.js PR has been approved, so the documentation PR can be merged. thank you!