[CHORE] Add a towncrier "deprecation" changelog type

Sub-issue of #639 — surface 3 of 3 ("What flows where"), manual path.

Scope

Give deprecations their own changelog section, so a PR that deprecates a field or model ships a <pr>.deprecation.md fragment like any other change.

Independent of every other sub-issue: it touches only the towncrier config and the contributor docs, and needs neither the extraction carrier nor any generated output.

Work

• Add a [[tool.towncrier.type]] block for deprecation to the root pyproject.toml ([tool.towncrier] at ~102, existing types at ~109-132). Place it after breaking so deprecations lead the notes ahead of features.
• Document the type in the changelog quick-start (docs/versioning.md) alongside breaking / feature / bugfix / docs / misc, and in the changelog-fragment guidance in CONTRIBUTING.md.

Acceptance

• towncrier build renders a <pr>.deprecation.md fragment under its own heading, in the intended position.
• The Require changelog on package change CI check accepts deprecation as a fragment type.

Follow-on

Automatic fragment generation — a codegen deprecation-manifest target that diffs against the last released tag and writes these fragments itself — is #677. This issue ships regardless of whether that one does.

Start with the [tool.towncrier] configuration and existing type blocks in pyproject.toml, then read the changelog quick-start in docs/versioning.md and fragment guidance in CONTRIBUTING.md. Add the deprecation type in the requested position and document it consistently. Run towncrier build and verify that a .deprecation.md fragment renders under its own heading and passes the changelog CI check.