Books that are "part of the stack" include these files by looking at their
branch. So the docs for Elasticsearch can use this:

```
include::{asciidoc-dir}/../../shared/versions/stack/{source_branch}.asciidoc[]
```

Which will resolve to the `6.3.asciidoc` file for the 6.3 branch. Or the
`7.2.asciidoc` file for the 7.2 branch. Or `master.asciidoc` for the master
branch.

NOTE: `current.asciidoc` is never automatically used.

Books outside of the stack have two choices:

1. Use the `current.asciidoc` file.
2. "Pin" theselves to some version of the stack.

Using `current.asciidoc` is useful because we'll update it whenever we release
a new version of the stack so the doc will automatically update on release.

"Pinning" the version of the stack that a book refers to is quite useful for
old versions of books for which we consistently see broken links when we release
new versions of the docs. It could also be useful for books that need special
care when bumping the stack version.