[DOCS] Rules updates (7.12)

[jmikell821]

Addresses:

• [DOCS] Threshold rules: multiple aggregations with cardinality #535
• [DOCS] Document to set limitations on what we support for Indicator Match rules in 7.12 #551
• [DOCS] [Security Solution] Update Compatibility with Cold Tier Nodes Documentation in 7.12.0 #562

[jmikell821]

Preview links:

• Cold tier nodes compatibility on the Elastic Security Overview and Detections/Alerts intro page
• Limited support for indicator match rules on the Detections/Alerts intro page
• Tune indicator match rules on the Tune detection rules page
• New threshold aggs on the Create a detection rule page

[nastasha-solomon]

Looks good overall! Only suggested two minor edits--and they might ultimately be unnecessary.

[MikePaquette]

@jmikell821 thanks for getting these changes in!

1. There seems to be a line-wrap problem in Cold Tier nodes compatibility section