…tic#56842)

This adds an API for simulating template composition with or without an index template.

It looks like:

```
POST /_index_template/_simulate/my-template
```

To simulate a template named `my-template` that already exists, or, to simulate a template that does
not already exist:

```
POST /_index_template/_simulate
{
  "index_patterns": ["my-index"]
  "composed_of": ["ct1", "ct2"],
}
```

This is related to elastic#55686, which adds an API to simulate composition based on an index name (hence
the `_simulate_index` vs `_simulate`).

This commit also adds reference documentation for both simulation APIs.

Relates to elastic#53101
Resolves elastic#56390
Resolves elastic#56255