The `bin` and `vendor` directories contain a fully checked out copy of
asciidoctor. You should be able to update it by changing the `Gemfile` and
running:

```
cd resources/asciidoctor
BUNDLE_APP_CONFIG=.bundle/prod bundle install --standalone --no-cache
```

The `lib` directory contains extensions to asciidoctor that are required for
Elastic's docs. You can test them with:

```
cd resources/asciidoctor
bundle install
./build/bin/rspec
```
