Add a build script for publication and use a plain DocC bundle (#94)

Adds a separate script that generates a version of the book suitable for
publishing on swift.org (with the non-staging header).

This conditionalizes which header.html is used.

This PR also moves away from using the package plugin to generate the
book and instead uses plain DocC commands. This simplifies the file
structure of the repo.

GitHub encountered conflicts that it couldn't resolve in the following
files, but which the Git "ort" merge strategy was able to resolve
automatically:

    TSPL.docc/LanguageGuide/StringsAndCharacters.md
    TSPL.docc/RevisionHistory/RevisionHistory.md

Author: amartini51

.gitignore

@@ -1,5 +1,7 @@
 .DS_Store
+.docc-build
 .swiftpm
 /.build
 /Package.resolved
-.docc-build
+/TSPL.docc/header.html
+/swift-book