* Remove `es-test-dir` book-scoped variable
* Remove `plugins-examples-dir` book-scoped variable
* Remove `:dependencies-dir:` and `:xes-repo-dir:` book-scoped variables
- In `index.asciidoc`, two variables (`:dependencies-dir:` and `:xes-repo-dir:`) were removed.
- In `sql/index.asciidoc`, the `:sql-tests:` path was updated to fuller path
- In `esql/index.asciidoc`, the `:esql-tests:` path was updated idem
* Replace `es-repo-dir` with `es-ref-dir`
* Move `:include-xpack: true` to few files that use it, remove from index.asciidoc
Specifying `?master_timeout=-1` on an API which performs a cluster state
update means that the cluster state update task will never time out
while waiting in the pending tasks queue. However this parameter is also
re-used in a few places where a timeout of `-1` means something else,
typically to timeout immediately. This commit fixes those places so that
`?master_timeout=-1` consistently means to wait forever.
Makes several changes to consolidate snapshot and backup-related docs.
Highlights:
* Adds info about supported ESS snapshot repository types
* Adds docs for Kibana's Snapshot and Restore feature
* Combines tutorial pages related to taking and managing snapshots
* Consolidates explanations of the snapshot process
* Incorporates SLM into the snapshot tutorial
* Removes duplicate "back up a cluster" pages
