Add extensible reference for [epic:123]

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What does this MR do and why?

Addresses #352864 (closed) by:

• Adding [epic:123] reference syntax ✅
• Adding [epic:123] reference syntax autocomplete ✅
• Updating documentation related to the new [epic:123] reference syntax ✅

References

Screenshots or screen recordings

Before	After
featureDisabled	featureEnabled

How to set up and validate locally

1. Ensure EE features are available (epics).
2. Enable extensible_reference_filters feature flag (http://localhost:3000/rails/features/).
3. Reference epic with [epic:number], [epic:number]+, or [epic:number]+s. Autocomplete should be available on RTE and PTE.

MR acceptance checklist

Evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

Related to #352864 (closed)