[DOCS] Update API Documentation for MITRE ATT&CK subtechniques

[spong]

Description

With the addition of MITRE Subtechniques in 7.11 (elastic/kibana#83511), the rule creation docs were updated (#432), but looks like we still need to update the API Documentation still.

@dplumlee, could you work with @jmikell821 and @nastasha-solomon on documenting this change please?

As surfaced on Discuss.

[jmikell821]

|Name |Type |Description

|framework |String |Relevant attack framework.

|tactic |Object a|Object containing information on the attack type:

* `id` - string, required
* `name` - string, required
* `reference` - string, required

|technique |Array a|Array containing information on the attack techniques (optional): 

* `id` - string, required
* `name` - string, required
* `reference` - string, required
* `subtechnique` - Array, optional

|subtechnique |Array a|Array containing more specific, information on the attack technique:

* `id` - string, required
* `name` - string, required
* `reference` - string, required`

[jmikell821]

Merged #668.