docs: add crate changelogs

[Tim-Zhang]

Summary

• add complete release histories for ttrpc, ttrpc-compiler, and
  ttrpc-codegen
• highlight API changes and migration guidance at minor release boundaries
• update the release checklist to include dated changelog entries

Why

Users currently have to reconstruct compatibility and upgrade information
from Git history. These changelogs make notable changes, yanked releases,
and API migrations discoverable in one place.

This PR changes documentation only; it does not modify Rust code.

Validation

• checked published versions, dates, and yanked status against crates.io
• validated release comparison commits and changelog references
• ran git diff --check

[Copilot]

Pull request overview

This PR adds reconstructed, dated changelogs for the ttrpc, ttrpc-compiler, and ttrpc-codegen crates, and updates the release checklist to require adding changelog entries as part of each release. This improves upgrade/migration discoverability without changing any Rust code.

Changes:

• Added full historical CHANGELOG.md files for ttrpc, ttrpc-compiler, and ttrpc-codegen, including API change notes and yanked-release annotations.
• Added migration guidance at minor release boundaries where API behavior changes.
• Updated RELEASE.md to require adding a dated changelog section and fixed/clarified release-step wording.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 1 comment.

File	Description
CHANGELOG.md	Adds the ttrpc crate’s full release history with API-change and migration notes.
compiler/CHANGELOG.md	Adds the ttrpc-compiler crate’s full release history with yanked-release annotations and API notes.
ttrpc-codegen/CHANGELOG.md	Adds the ttrpc-codegen crate’s full release history with API notes and upgrade guidance.
RELEASE.md	Updates the release checklist to require dated changelog entries and clarifies publishing steps.

Suppressed comments (1)

RELEASE.md:28

• This sentence introduces an ordered sub-list; ending it with a colon improves readability and matches common Markdown style.

5. If the dry run succeeds, publish the crates that need publishing using `cargo publish -p <crate>` in the following order

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[Copilot]

Pull request overview

Copilot reviewed 4 out of 4 changed files in this pull request and generated no new comments.