Document community adoption

Take the following steps within the specification: Link to the cql Zulip stream on chat.fhir.org Document language capabilities and expand conformance level descriptions Include discussion of known domain applications of CQL Include examples from each domain Language Capabilities Language Capabilities Data Model and Type Support Decimal precision and scale Datetime precision and scale UCUM unit conversion support Locale support Regex dialect  # Supported data models Compatibility Level  ## 1.3 System.Ratio System.Date Expand operator FHIRPath Support  ## 1.4 Precision operators for Decimal and Date/Time types Model-defined contexts Library namespaces Related-context Retrieves Unit conversion support  ## 1.5 Modular Arithmetic for Quantities Expand a single interval Aggregate clause Include Retrieves Searchpath Retrieves Id Retrieves System.Long System.Vocabulary System.CodeSystem System.ValueSet System Data Types Operators Logical Operators Type Operators Nullological Operators Comparison Operators Arithmetic Operators String Operators Date and Time Operators Interval Operators List Operators Aggregate Functions Clinical Operators Diagnostic Operators Queries Single Source Relationships Let Clause Where Clause Return Clause Aggregate Clause Multi Source Retrieve Unfiltered Context Terminology-based Retrieve Valueset Terminology target CodeSystem Terminology target Searchpath Retrieve Include Retrieve Related-context Retrieve Profile-based Retrieve