Create "current" stack version file

shared/versions/stack/README

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+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 he 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.

shared/versions/stack/current.asciidoc

@@ -0,0 +1 @@
+include::versions/stack/7.4.asciidoc[]