docs: document Oathkeeper bounded retry controls

[aeneasr]

Documents the opt-in bounded retry policy introduced by ory-corp/cloud#13007 for Oathkeeper's external HTTP pipeline handlers.

The update:

• defines the shared retry contract for the OAuth2 client credentials and introspection authenticators, remote and remote JSON authorizers, and hydrator mutator;
• documents required fields, defaults, validation, deadlines, per-attempt timeouts, attempt limits, 429 handling, and deprecated legacy behavior;
• updates all five handler examples to use the bounded policy; and
• corrects the hydrator configuration path from api.auth.retry.* to api.retry.

Validation

• npm exec -- prettier --check docs/oathkeeper/pipeline.md docs/oathkeeper/pipeline/authn.md docs/oathkeeper/pipeline/authz.md docs/oathkeeper/pipeline/mutator.md
• npm run build (successful; reports existing unrelated broken-link warnings)

Related implementation

• ory-corp/cloud#13007