Flag warnings introduced by a PR by running a CI check

[Kyle-Ye]

After #1, #36 and https://github.com/apple/swift-docc/issues/377 were resolved, currently there should be no warnings when building the newest swift-book using the newest swift-docc.

Input:
    /Users/kyle/Workspace/Github/swift-docc-project/swift-book/Sources/TSPL/TSPL.docc
Template:
    /Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault
    .xctoolchain/usr/bin/../share/docc/render
==============================
Starting Local Preview Server
    Address: http://localhost:8080/
==============================
Monitoring
    /Users/kyle/Workspace/Github/swift-docc-project/swift-book/Sources/TSPL/TSPL.docc

Maybe we should consider to config a CI check to make sure a future PR will not produce any new warning or error(with --warnings-as-errors enabled on swift-docc side).

https://forums.swift.org/t/add-support-for-warnings-as-errors/59832/5