docs: add ADR for persisted competency deletability signal

[mgwozdz-unicon]

Why

openedx-core#655 proposes how to enforce the
delete/edit guardrails ADR-0002 and ADR-0003 already decided for competency taxonomy data, and
raised surfacing a deletable signal on the relevant GET endpoints so Studio can disable/warn on
the delete action instead of only discovering the block from a failed request. How to compute that
signal turned into a real design question (read-time cost vs. write-path complexity), so it gets
its own ADR rather than staying an implementation detail of that issue.

Summary

• Adds ADR 0005: persist a deletable boolean on the five delete-protected models (Taxonomy,
  CompetencyTaxonomy, Tag, ObjectTag, CompetencyCriteriaGroup, CompetencyCriteria) rather
  than computing it per-request or caching it.
• The flip is one-directional (True to False only, never back), set synchronously in the same
  transaction as the learner-status write that triggers it. No cache-invalidation logic needed as a
  result.
• No backfill migration needed: the referencing learner-status tables are new, so no existing row
  can already be non-deletable.
• Enforcement stays on the write path; the persisted field is a UX convenience, not the guardrail
  itself.
• Rejects two alternatives considered in [Arch] Implementation approach for competency data delete/edit guardrails #655: a per-request Exists()/OuterRef() annotation, and
  caching that computed value with a TTL.

Related to #655.

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