Remove Jazzy doc generation (#191)

Motivation:

Docs are generated by and hosted on the Swift Package Index. We no
longer need the script to generate docs via Jazzy.

Modifications:

- Remove the generate_docs script
- Remove Jazzy from the Dockerfile but keep Ruby; it's used for
  generating test manifests.
- Remove SwiftFormat from the Dockerfile; we don't use it.

Result:

Fewer unused things.

docker/Dockerfile

@@ -17,19 +17,9 @@ RUN apt-get update && apt-get install -y wget
 RUN apt-get update && apt-get install -y lsof dnsutils netcat-openbsd net-tools curl jq # used by integration tests
 RUN apt-get update && apt-get install -y zlib1g-dev
 
-# ruby and jazzy for docs generation
+# ruby for soundness
 RUN apt-get update && apt-get install -y ruby ruby-dev libsqlite3-dev build-essential
-# jazzy no longer works on xenial as ruby is too old.
-RUN if [ "${ubuntu_version}" = "focal" ] ; then echo "gem: --no-document" > ~/.gemrc ; fi
-RUN if [ "${ubuntu_version}" = "focal" ] ; then gem install jazzy ; fi
 
 # tools
 RUN mkdir -p $HOME/.tools
 RUN echo 'export PATH="$HOME/.tools:$PATH"' >> $HOME/.profile
-
-# swiftformat (until part of the toolchain)
-
-ARG swiftformat_version=0.40.12
-RUN git clone --branch $swiftformat_version --depth 1 https://github.com/nicklockwood/SwiftFormat $HOME/.tools/swift-format
-RUN cd $HOME/.tools/swift-format && swift build -c release
-RUN ln -s $HOME/.tools/swift-format/.build/release/swiftformat $HOME/.tools/swiftformat